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Lazy loading images with accessibility and printer support

I am looking for a proper way to implement lazy loading of images without harming printability and accessibility, and without introducing layout shift (content jump), preferrably using native loading=lazy and a fallback for older browsers. Answers to the question How lazy loading images using JavaScript works?
included various solutions none of which completely satisfy all of these requirements.
An elegant solution should be based on valid and complete html markup, i.e. using <img src, srcset, sizes, width, height, and loading attributes instead of putting the data into data- attributes, like the popular javascript libraries lazysizes and vanilla-lazyload do. There should be no need to use <noscript> elements either.
Due to a bug in chrome, the first browser to support native lazyloading, images that have not yet been loaded will be missing in the printed page.
Both javascript libraries mentioned above, require either invalid markup without any src attribute at all, or an empty or low quality placeholder (LQIP), while the src data is put into data-src instead, and srcset data put into data-srcset, all of which only works with javascript. Is this considered an acceptable or even best practice in 2020, and does this neither harm the site accessibility, cross-device compatibility, nor search engine optimization?
Update:
I tried a workaround for the printing bug using only HTML and CSS #media print background images in this codepen . Even if this worked as intended, there would be a necessary css directive for each and every image, which is neither elegant nor generic. Unfortunately there is no way to use media queries inside the <picture> element either.
There is another workaround by Houssein Djirdeh at at lazy-load-with-print-ctl1l4wu1.now.sh using javascript to change loading=lazy to loading=eager when a "print" button is clicked. The same function could also be used onbeforeprint.
I made a codepen using lazysizes.
I made another codepen using vanilla-lazyload .
I thought about forking a javascript solution to make it work using src and srcset, but this must probably have been tried before, the tradeoff would be that once the lazyloading script starts to act on the image elements, the browser might have already started downloading the source files.

Just show me your hideous code, I don't want to read!
If you don't want to read my ramblings the final section "Demo" contains a fiddle you can investigate (commented reasonably well in the code) with instructions.
Or there is a link to the demo on a domain I control here that is easier to test against if you want to use that.
There is also a version that nearly works in IE here, for some reason the "preparing for print" screen doesn't disappear before printing but all other functionality works (surprisingly)!
Things to try:
Try it at different browser sizes to see the dynamic image requesting
try it on a slower connection and check the network tab to see the lazy loading in action and the dynamic change in how lazy loading works depending on connection speed.
try pressing CTRL + P when the network connection is slow (without scrolling the page) to see how we load in images not yet in the DOM before printing
try loading the page with a slow network connection and then using FILE > PRINT to see how we handle images that have not yet loaded in that scenario.
Version 0.1, proof of concept
So there is still a long way to go, but I thought I would share my solution so far.
It is complex (and flawed) but it is about 90% of what you asked for and potentially a better solution than current image lazy loading.
Also as I am awful at writing clean JS when prototyping an idea. I can only apologise to any of you brave enough to try and understand my code at this stage!
only tested in chrome - so as you can imagine it might not work in other browsers, especially as grabbing the content of a <noscript> tag is notoriously inconsistent. However eventually I hope this will be a production ready solution.
Finally it was too much work to build an API at this stage, so for the image resizing I utilised https://placehold.it - so there are a few lines of redundant code to be removed there.
Key features / Benefits
No wasted image bytes
This solution calculates the actual size of the image to be requested. So instead of adding breakpoints in something like a <picture> element we actually say we want an image that is 427px wide (for example).
This obviously requires a server-side image resizing solution (which is beyond the scope of a stack overflow answer) but the benefits are massive.
First of all if you change all of your breakpoints on the site it doesn't matter, so no updating picture elements everywhere.
Secondly the difference between a 320px and 400px wide image in terms of kb is over 40% so picking a "similarly sized" image is not ideal (which is basically what the <picture> element does).
Thirdly if people (like me) have massive 4K monitors and a decent connection speed then you can actually serve them a 4K image (although connection speed detection is an improvement I need to make in version 0.2).
Fourthly, what if an image is 50% width of it's parent container at one screen size, 25% width of it's parent container at another, but the container is 60% screen width at one screen size and 80% screen width at another.
Trying to get this right in a <picture> element can be frustrating at best. It is even worse if you then decide to change the layout as you have to recalculate all of the width percentages etc.
Finally this saves time when crafting pages / would work well with a CMS as you don't need to teach someone how to set breakpoints on an image (as I have yet to see a CMS handle this better than just setting the breakpoints as if every image is full width on the screen).
Minimal Markup (and semantically correct markup)
Although you wanted to not use <noscript> and avoid data attributes I needed to use both.
However the markup you write / generate is literally an <img> element written how you normally would wrapped in a <noscript> tag.
Once an image has fully loaded all clutter is removed so your DOM is left with just an <img> element.
If you ever want to replace the solution (if browser technology improves etc.) then a simple replace on the <noscripts> would get you to a standard HTML markup ready for improving.
WebP
Of course this solution requests WebP images if supported (its all about performance!). On the server side you would need to process these accordingly (for example if an image is a PNG with transparency you send that back even if a WebP image is requested).
Printing
Oh this was a fun one!
There is nothing we can do if we send a document to print and an image has not loaded yet, I tried all sorts of hacks (such as setting background images) but it just isn't possible (or I am not clever enough to work it out....more likely!)
So what I have done is think of real world scenarios and cover them as gracefully as possible.
If the user is on a fast connection we lazy load the images, but we don't wait for scroll to do this. This could mean a bit more load on our servers but I am acting like printing is highly important (second only to speed).
If the user is on a slow connection then we use traditional lazy loading.
If they press CTRL + P we intercept the print command and display a message while the images are loading. This concept is taken from the example OP gave by Houssein Djirdeh but using our lazy loading mechanism.
If a user prints using FILE > PRINT then we instead display a placeholder for images that have not yet loaded explaining that they need to scroll the page to display the image. (the placeholders are approximately the same size as the image will be).
This is the best compromise I could think of for now.
No layout shifts (assuming content to be lazy loaded is off-screen on page load).
Not a 100% perfect solution for this but as "above the fold" content shouldn't be lazy loaded and 95% of page visits start at the top of the page it is a reasonable compromise.
We use a blank SVG (created at the correct proportions "on the fly") using a data URI as a placeholder for the image and then swap the src when we need to load an image. This avoids network requests and ensures that when the image loads there is no Layout Shift.
This also means the page is semantically correct at all times, no empty hrefs etc.
The layout shifts occur if a user has already scrolled the page and then reloads. This is because the <img> elements are created via JavaScript (unless JavaScript is disabled in which case the image displays from the <noscript> version of the image). So they don't exist in the DOM as it is parsed.
This is avoidable but requires compromises elsewhere so I have taken this as an acceptable hit for now.
Works without JavaScript and clean markup
The original markup is simply an image inside a <noscript> tag. No custom markup or data-attributes etc.
The markup I have gone with is:
<noscript class="lazy">
<img src="https://placehold.it/1500x500" alt="an image" width="1500px" height="500px"/>
</noscript>
It doesn't get much more standard and clean as that, it doesn't even need the class="lazy" if you don't use <noscript> tags elsewhere, it is purely for collisions.
You could even omit the width and height attributes if you didn't care about Layout Shift but as Cumulative Layout Shift (CLS) is a Core Web Vital I wouldn't recommend it.
Accessibility
The images are just standard images and alt attributes are carried over.
I even added an additional check that if alt attributes are empty / missing a big red border is added to the image via a CSS class.
Issues / compromises
Layout Shift if page already scrolled
As mentioned previously if a page is already scrolled then there will be massive layout shifts similar to if a standard image was added to a page without width and height attributes.
Accessibility
Although the image solution itself is accessible the screen that appears when pressing CTRL + P is not. This is pure laziness on my part and easy to resolve once a more final solution exists.
The lack of Internet Explorer support (see below) however is a big accessibility issue.
IE
UPDATE
There is a version that nearly works in IE11 here. I am investigating if I can get this to work all the way back to IE9.
Also tested in Firefox, Edge and Safari (mobile), seems to work there.
ORIGINAL
Although this isn't tested in Firefox, Safari etc. it is easy enough to get to work there if there are issues.
However accessing the content of <noscript> tags is notoriously difficult (and impossible in some versions) in IE and other older browsers and as such this solution will probably never work in IE.
This is important when it comes to accessibility as a lot of screen reader users rely on IE as it works well with JAWS.
The solution I have in mind is to use User Agent sniffing on the server and serve different markup and JavaScript, but that is complex and very niche so I am not going to do that within this answer.
Checking Latency
I am using a rather crude way of checking latency (to try and guess if someone is on a 3G / 4G connection) of downloading a tiny image twice and measuring the load time.
2 unneeded network requests is not ideal when trying to go for maximum performance (not due to the 100bytes I download, but due to the delay on high latency connections before initialising things).
This needs a complete rethink but it will do for now while I work on other bits.
Demo
Couldn't use an inline fiddle due to character count limitation of 30,000 characters!
So here is the current JS Fiddle - https://jsfiddle.net/9d5qs6ba/.
Alternatively as mentioned previously the demo can be viewed and tested more easily on a domain I control at https://inhu.co/so/image-concept.php.
I know it isn't the "done thing" linking to your own domains but it is difficult to test printing on a jsfiddle etc.

The proper solution for printable lazy loading in 2022 is using the native loading attribute.
<img loading=lazy>
The recommendation to use a custom print button has been obsoleted as chromium issue 875403 got fixed.
Prior recommendations included adding a custom print button (which did not fix the problem when using the native browser print functionality) or using JavaScript to load images onBeforePrint the latter not being considered a good solution, as loading=lazy, as a "DOM-only" solution, must not rely on JavaScript.
Beware that, even after the bug fix, some of your users might still visit your site with a buggy browser version.

#Ingo Steinke Before one dwells into answers for the concerns that you have raised, one has to go back and think about why lazy loading came about and for what detriment it solved on initiation as framework of thought. Keyword framework of thought... it is not a solution and I would go on a leaf to say it has never been a solution but framework of thought.
Why we wanted it:
Minimise unnecessary file fetching from server - this is bandwidth critical if one is running a large user base. So it was the internet version of just in time as in industrial production.
Legacy browser versions and before async and defer were popularised in JS/HTML, interactivity with the browser window remained hampered until all content was loaded.
Now broad band as we know it has only been around since the last 6-7 years in real sense of manner and penetration. We wanted it because we didn't want to encounter no.2 on low bandwidth. To be honest, there was and still is a growing concern and ideology of minifying and zipping JS and CSS files - all because that round trip to server and back should be minimised so that next item in the list could be fetched. Do keep in mind browsers tend to limit simultaneous downloading connections to around 6 at a time per window or active window. There is reasons why Google popularised the 3 second rule. If above were to let run on as it than 3 second rule will fall on its head as if it did not have legs.
So came along thought frameworks.
Image as CSS background: This came as it did not mess up the visual aspect of the page. Everything remained as it is in its place and then suddenly became colourful. It was time when web pages seemed to have elastic fit i.e. it was that bag which once filled with air suddenly poped-transformed into jumping castle. This was increasingly become bad idea as front end developer. So fixing height and with of the container then run images as background helped and HTML5 background alignment properties upgraded them self accordingly. There was even variant and still used as in use multiple backgrounds one being loading spiril or low end blured image version on top of which actual intended image was fetched. Since level down bacground would be fetched and populated everywhere in single instance of downloading it created a more pleasing visual and user knew what to expect. worked in printing as well even if intended image did not download.
Then came JS version of it by hijacking DOM either through data-src, invalid image tags removing src, and what not. only trigger the change when content is scrolled to. Obviously there would be lag but that was either countered through CSS approach implemented in JS or calculating scroll points and triggering event couple of pixel ahead. They all still work on the same premise.
There is one question that begs to be asked and you have touched it in your pretext .... none of it controls or alters browser native functionality. Browser might as will go fetch the item even before your script had anything to do with any thing.
This is the main issue here. BOM does not care and even want to care about what your script is asking to do all it knows if there is a src property fetch the content. None of the solutions have changed that. If we could change that functionality then thought framework would become solution.
I still believe browsers should not change that just for the sake of it and thus never gained tracking in debates. What browsers have done is pre-fetching known as speculative or look-ahead pre-parser, It is the single biggest improvement in browsers that deserves it credit. Just as we type url in address bar on every chnage of string browser is pre-fetching the content even though I had not typed the whole url. I had specially developed a programme where I grabbed anything that was received at server from these look-ahead pre-parsers. It takes less than second to get response at most times and browsers begin to process it all including images and JS. This was counter the jerky delayed elastic prone display as discussed in No.1 and No.2. It did not reduce the server hit however. The reason why we are doing lazy loading any ways. But some JS workaround gained traction as there was no src property so pre-parser did not fetch the image and was only done so when user actually sent to the page and events were triggered. Some browser have toyed with the idea of lazy loading them self but let go if it as it did not assume universal consistency in standard.
Universal Standard is simple if there is src property browser will fetch the item no if and buts. Imagine if that was not the case OMG hell would break loose on poor front-end developer.
So deep down what you are raising in debate is the question regarding BOM functionality as I have discussed above. There is no work around for it. In your case both for screen and print version of display. How to make sure images are loaded when print command is sent. Answer is simple for BOM print is after the fact. Fact ebing screen display and before the fact being everything before that at BOM/DOM propagation level. Again you cannot change that.
So you have to make trade off. Trade off would come in the form of another thought framework. rather than assuming everything is print ready make it print ready on user command. There is div that pops up and shows printed version of document and then print from there on. UI could be anything it would only take second or so as majority of the content would be loaded any ways and rest will take short amount of time. CSS rules for print could mighty handy in this respect. You can almost see it in action in may places on the internet.
conclusion as we stand today where we are with BOM functionality bundling the screen display and print display with lazy load is not what lazy lading was intended for thus does not provide any better solution then mere hacks. So you have to create your UI based on your context separating the two, to make it work properly.

HTML Image map vs. multiple div elements

I am implementing an Angular component which has many little areas where user can click.
I see 2 ways how to do it:
use background image and define map areas where user can click, add click handler on the image.
use div for each clickable part and attach the click handler to parent element (so I don't have many click handlers on each div). Use the CSS to style each div so it looks like the image in 1.
The problem with 1. is that when the image changes I have to change the map coordinates too.
The advantage here is that it should work without problem in many browsers.
The advantage of the 2. is that I can style the component as I need (so it can be smaller, bigger, different font, etc.), but it can have performance impact when I need to show more than one such component.
Which way would you choose and why? Or are there any other possibilities?
Edit:
forgot to mention that the component should also work on mobile devices

I don't remember where but i just read a nice article about:
div(webkit-transform) vs svg vs canvas
To make it simple:
1-50 elements = divs & images
50-500 elements = svg
500 and more elements = canvas
this are here just to give you an idea ...then everything depends on every element and device.
Canvas would be the best solution for everything.
The problem on canvas is the click handler.You need to create a collision detection script.
(i used a canvas with a background image(worldmap),to show the dots in real time of the current users.).but it's not clickable... (there is a clickable legend under/over the map)
SVG is prbably the best solution in your case
like #mainguy said u can draw stuff and add eventHandlers (or one like on the parent element).and the performance is better than divs.
DIVS
Most of the time i use div sets with one eventhanlder. they are so easy to use and style.. but only squares or circles.. and if you start to style them you loose alot performance (box-shadow..).
If you don't style the divs you can use alot of them.Especially if you put the eventhandler on the parentNode.
that way you can handle 1000's of elements without problems.(but don't use position:absolute)
Image Map
Again ... if there are not too many elements this is prolly also a good solution.. the simplest ... (the simplest way to draw your simple shapes).As soon you have you static MAP values you can then transform your imagesize recalculation the map with the ratio.. so thats not a problem.
I would go for the image map if there are not to many elements.
else SVG.
Everything depends on how many elements you have.
is that when the image changes I have to change the map coordinates too
if you use divs you don't have to change coords?
** Mobile devices support more than ie browsers.

I would do this with a SVG library which are fast, easy to style and suporrted by most browsers.
D3.js
is available for Angular as directive. Have not worked with his one yet. But it seems to be very popular. Just look on their homepage for a WOW! effect.
Raphaël JS
Was used by me in many projects. It has the big advantage that it supports even old IEs (sic!).
Just make a search for Angular SVG and/or JS and you will find a lot of solutions.

Not knowing the exact requirements, my default choice would be 2.
There are drawbacks, such us inability to easily map non-rectangular areas. The advantages however are huge, including easy maintenance, possible responsiveness, more robust styling possibilities and more.
The sole amount of divs shouldn't cause any problems and I wouldn't worry about it.
There are cases in which I would look for different implementation, though.

Text masks out the background

You have an object (span or div e.g.) with text. And this text should mask out the colored background of the object so that you can see the very background (of body element e.g.).
I know there is "mask-image". But they don't want to develop this CSS3 technique further. Is there a Javascript solution maybe so anyone can see this?
Is there any chance to do this cross-browser?

Can't think of a technique to do this in javascript or html. However, an approach that could be used (although not entirely desirable - I realize that this is going to be time consuming) would be to create a transparent .png for each letter, set the background of the letter to transparent and the rest of the image to some color, and then use those. You would have to make a whole set for each color you wished to use. You could also just make an image which was one word in a .png and use that which would be easier. This would be the only current way to get this to work with all browsers including older ones such as IE6 where the newer techniques are not available.
The reason, in my opinion, that this will not currently work, is that when you have text inside of a <div> or <span> even if the text is transparent, it would merely default to the color of the <div> or <span>. If their background was set to transparent, then nothing would even show up. Conversely, if their color was set, the text would not show up because it would merely inherit that color.

It looks like the closest you can get to full cross-browser support (without using images for each character in the text) is the very-limited tricks shown here. These tricks wouldn't work with a photographic bg image though.
If you don't require support for IE8 or earlier, you could investigate doing this in SVG or Canvas. Note however that Android prior to 3.0 doesn't support SVG, which rules out as much as 85% of existing Android devices.
Webkit has a proprietary background-clip:text style, but it only works on Chrome and Safari.
Using color:transparent for the text turned out to be completely useless for this.
On the whole, the best bet may be canvas, especially if you can find a JS library or jQuery plug-in that provides a VML alternative for IE8 and earlier (assuming the VML solution supports text masks), or if you can provide a graceful-degradation fallback option for IE8 and earlier that looks adequate even without a text mask.
Another solution that's easily overlooked: If you can dynamically render an image on the server using the current text on the page (caching it as needed), and then serve that image to the client, then there are no cross-browser issues to deal with. This could possibly be done in PHP using the GD graphics library, for instance.

SVG support for Internet Explorer 8 and below

I created animation in flash and converted it into HTML5 using Swiffy.
I think it's using SVG to render all of it; is there some JavaScript or a trick to make IE8 and below support it? My animation is working well with Internet Explorer 9.
Thanks!

While no option is perfect, there are a few choices:
1.) Adobe has a SVG plugin for IE8 http://www.iegallery.com/en/addons/detail.aspx?id=444
2.) The Raphael JavaScript Framework allows vector graphics cross browser - http://raphaeljs.com/
3.) Then there are the Open Source projects: http://code.google.com/p/svg2vml/ and http://code.google.com/p/svgweb/
4.) There is the option to display flash for IE8 and below.
5.) There is the option to gracefully degrade for IE8 and below and show a static image in place of the animation.
Based on your reason for the animation - I would recomend 4 or 5.

There are two options that will not tie you to specific frameworks (raphael) or complicated solutions (svgweb):
Chrome Frame: if you're going to get an extension, get that one
Server-side rasterization: send your SVG back to the server, inkscape rasterize to png, send it back.
If you go number 2 (yes it is a crappy option), and want to keep clickable parts and tooltips, you will have to use invisible divs, or labels that are in HTML on top of the rendered svg.
You can reuse the positioning information from the svg to position those divs in order to avoid overhead for that part.
The best option probably is to diplomatically encourage them to use a real browser, but then it's not always possible for those who live under the rule of an unskilled IT department ;)

Drawing arbitary pictures on a webpage

I need to display a runtime-generated image (mostly consisting of nice-looking boxes, lines and text) on a webpage. We're currently using ASP.NET MVC3.
The problem is, I haven't really done web development before, and I have no idea how to go about drawing an arbitary diagram on a webpage. Do I use some sort of javascript? How, and what do I use? Do I generate the image on the server somehow and simply display it? Something else?
I have no idea where to even start, let alone solve the actual problem...

It depends on the target browser.
If you target the most modern browsers, you can use an HTML5 Canvas and draw on it using JavaScript (see http://www.williammalone.com/articles/create-html5-canvas-javascript-drawing-app/).
If you're targeting other browsers (as well), the easiest is probably to generate an image on the server side (GIF/JPG/PNG). Your HTML page then contains an IMG tag with the SRC parameter set to the page that generates and returns the image. You can find an example at this page: http://www.sitepoint.com/generating-asp-net-images-fly/ (this uses ASP.NET to generate the image).

Roy Dictus fairly well described the overall approach you'll want to take. If you decide to target modern browsers and use javascript, I'd suggest you look at Processing.js. It's a javascript port of a popular Java library, and the web page has some code samples to show how you can create both static drawings and animations with some simple code.