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HTML - CSS - How to left-align images in a centered and resizable parent

I have a lot of thumbnails in my page: I want them to be centered, but the last line looks awful while resizing and I have something like one or two orphan images floating in the center of the page.
The last line should be left-aligned.
How to do it?
I tried to insert another div with "margin:0 auto 0 auto;" but it doesn't work.
https://jsfiddle.net/4hw4fkm9/
What I try to have:
<div style="text-align:center;">
<img />
<img />
<img />
<img />
etc..
</div>

You should use CSS for this, not the HTML attribute "align", which is pretty old school. You can either float the images left, in which case they will stack to the left, or you could set all the images to display: inline-block, then use text-align: center on their container element (the div in this case).
Here are some references:
http://www.w3schools.com/css/css_float.asp
http://www.w3schools.com/cssref/pr_class_display.asp

I'm not sure if this is what you're looking for - as you mention that you both 'want them centered' but want them left-aligned, too.
What you can do is create another div that will act as a container, set width: 100% on the main div, set the width slightly lower on the container div and set margin: 0 auto; on the container. Then make the images relatively positioned with left:0;.
See Updated Fiddle Here. Is this what you're after?
EDIT: Re-Updated Fiddle for my attached comment. You can use set pixel values on the main and container div to account for the set pixel width of the thumbnails + any spacing that occurs.

Using the selectors in the fiddle.
.center {
text-align:center;
background-color:red;
margin:0 auto;
padding:0 20px;
}
.center:after{
content:'';
display:table;
clear:both;
}
img {
width: 100px;
height: 100px;
float:left;
}
From there the .center element would need a width of some multiple of 100px to include a fixed number of imgs. I added padding to make it look closer to the picture you provided.

Simulating a "Card Layout" when using Flexbox

I'm having a rather complex problem here. I'd like to make a Layout in CSS featuring multiple Layers which means basically having multiple <div> elements stacked ontop of each other (or at least make it look like this) and only the top one is visible.
In many other GUI Environements this is known as a Card Layout.
So the plan was:
Create all <div class='layer'> elements in one parent <div class='container'> element
Make them have display: none initially
invoke $.show() or $.hide() to show or hide them
HTML:
<div id="containerOne">
<div class="layer a"></div>
<div class="layer b">
<div class="inner b1"></div>
</div>
<div class="layer c"></div>
</div>
CSS:
#containerOne {
width: 150px;
height: 150px;
background: red;
}
#containerOne .layer {
display : none;
width: 100%;
height: 100%
}
.a {
background: green;
}
.b {
background: orange;
}
.b1 {
width: 50%;
height: 50%;
background: yellow;
}
.c {
background: blue;
}
This works at first since the elements will not have any space reserved on the page initially and will only occupy space when shown.
Now if i have a nested element inside a Layer and i want it to have a size of 50% x 50% this also works well: Even if the layer div is set to display: none initially.
Now by default my Container div is set to display: block and all the size calculation seems to work fine and here's where my problem begins:
I need the Container div to have display: flex to take advantage of the Flexbox features of modern browsers. Instead of setting a fixed width for my Layers i now configure them to have flex: 1 so they grow to the full size of the container.
This also works as expected. With one exception: The nested Element that should have a size of 50% x 50% won't have a size at all. Debugging this in JavaScript shows that this is because at the moment the Layers are created they will all have a size of 0 x 0 and will first get their size assigned when they are shown by $.show().
What i would expect is that once they are shown, the nested element will grow to its 50% x 50% size but it doesn't.
Is there anything i can do about it? (good solutions and back-hack-workaround solutions).
I also made a Fiddle to demonstrate the problem and to play around with.

The thing you're expecting to happen should happen, but it doesn't (in Chrome) due to a bug. It actually works correctly in Firefox.
Here's a simplified version of your testcase with no dynamic changes:
http://jsfiddle.net/CN7e8/4/
This behavior was actually recently changed in the flexbox spec, to the behavior you're expecting. The issue is that your 50% height on b1 is resolving against an auto-height element (the flex item, b) and Chrome is treating that auto-height as an invalid percent basis, even though the auto-height can actually be resolved to the container's height. (It'll become the container's height due to the default align-items: stretch on the flex container, which makes auto-height flex items take on the container's height).
For reference, the spec change to clarify this is mentioned under ISSUE 3, part (a), in this post:
http://lists.w3.org/Archives/Public/www-style/2014Mar/0350.html
ANYWAY. To work around the Chrome bug, you can't have an auto-height on your flex item b, given that you're depending on it being a percent-basis. You have to give it an explicit height, e.g. height:100% (which more directly resolves against the container's height). Here's your fiddle, with that changed: http://jsfiddle.net/CN7e8/5/

Image no longer responsive as put in table cell layout

I'm trying to center an image on a site. As I want it to be centered both horizontally and vertically I used a table/table-cell layout as following:
<div id="mainContainer>
<div id="horizon">
<img src="url">
</div>
</div>
Here mainContainer ist set to display: table and horizon is set to display: table-cell.
Unfortunately the image is no longer resizing proportionally as it becomes part of this structure. As soon as I move it into the table/table-cell divs it resizes to its original size (instead of being resized proportionally because of max-width: 100% and max-height: 100%)
see: http://jsfiddle.net/U8KcN/
EDIT:
My bad. I just tried to simplify the issue. I want to build a little slideshow for images. The problem here is that I do not know which sizes the images are (referring to someone else using the slideshow). So in simple terms I need a specific CSS that 1) centers the image vertically and horizontally in the div if its width and height are smaller than the div's dimensions, 2) resizes the image automatically if it is bigger than the div's dimensions (unnecessary to add, it is unknown whether it's width or height is bigger).
source: OP Comment

Every time I hear someone talk about the evils of using tables and then see them create a complete table structure out of DIVs with pretty much the same amount of markup -- sometimes more -- it really gets on my nerves.
By the way, have you ever developed a site for someone who wanted to do some of their own layouts in the content area using a CMS like Wordpress? The only way someone with almost no HTML knowledge can do that well is by allowing them to use table tools in Tiny MCE or another editor. So yes, tables are still important.
Recently, I had to do just that for a customer, but the images in the table cells were not responding and shrinking for smaller screen sizes. All I had to add to my CSS to care of that problem was:
table {
table-layout: fixed;
}
Now the images in the table cells are growing and shrinking accordingly, and the horizontal scrollbar is no longer appearing. Oh, of course you have to use percentages in your table width and td widths as well.

How about dropping that "CSS-table" stuff and doing it a bit easier?
<div style="width:auto;height:auto;margin:25%;text-align:center;vertical-align:middle">
<img src="URL">
</div>
At least, that’s how I would handle it...
EDIT:
Please note that I've put the CSS inline to show you what element should get what style. In production, you should — as a comment to this answer correctly stated — always separate style from code like. So, practically, you'll end up with something like this:
<style>
.centerimg {
width:auto;
height:auto;
margin:25%;
text-align:center;
vertical-align:middle
}
</style>
...
<div class="centerimg">
<img src="#">
</div>
EDIT 2:
Replying to the related comment, here's the update to make the image fit it's parent proportionally:
If your image has a bigger width than height...
<style>
...
img{
max-width:100%;
height:auto
}
</style>
or, if your image has a smaller width than height...
<style>
...
img{
max-height:100%;
width:auto
}
</style>
EDIT 3:
Looking at your fiddle, I came up with this which works like you want it to work:
<style>
*{
width:100%;
height:100%;
margin:0;
text-align:center;
vertical-align:middle
}
img{
width:auto;
height:100%;
}
</style>
I've forked your fiddle to show the updates: http://jsfiddle.net/LPrkb/1/
EDIT 3:
As the OP doesn't seem to be able to decide what he needs, I'm adding this final edit due to his latest comment.
You could also use a CSS background-image with "background-size:contain" on the "mainContainer" and be done with it... check http://jsfiddle.net/HGpfJ/2/ or look at this 100% working example taking a completely different approach which results in just the same effect/functionality:
<!DOCTYPE HTML>
<html itemscope="itemscope" itemtype="http://schema.org/WebPage">
<head>
<title>Example</title>
<style>
html,body{width:100%;height:100%;margin:0;padding:0}
#centerimg{
width:100%;
height:100%;
background:transparent url(http://oi42.tinypic.com/v9g8i.jpg) no-repeat scroll center;
background-size:contain;
}
</style>
<body>
<div id="centerimg"></div>
</body>
</html>
Let's face the facts: depending on where in the document structure you want to have the image "centered", there are more than a dozen of ways to do it.
If OP needs specific code, we will need the complete document structure from OP and not simply a "generalized" code-snippet which could be anywhere in whatever document structure.

Using tables (display: table-cell) for layout is a web development anti-pattern these days. Try using the <span> element with display: inline-block to vertically and horizontally center the image, as this method will work all the way back to IE 6. Also, you can change the image size to be a percentage if you want it to resize according to its container:
http://jsfiddle.net/hHWy8/1/
HTML:
<span class="horizontal">
<span class="vertical">
<img src="url" />
</span>
</span>
CSS:
span {
display: inline-block;
}
span.horizontal {
text-align: center;
width: 100%;
}
span.vertical {
vertical-align: middle;
border: 1px solid black; /* put a border on container for display purposes */
line-height: 1000px; /* this sets the vertical height */
overflow: hidden;
white-space: nowrap;
}
span.vertical img {
height: 50px; /* set your image height (could be percent) */
width: 50px; /* set your image width (could be percent) */
}
span.vertical br {
display: none;
}

To resize your image you could use this css:
html,body,div,img{
height:100%;
}
img { width:100%}
It sets the html and body's height to 100%, this is needed so the height of your page takes all available space

Overlaying one div over another, but not knowing the size of the div

I'm trying to lay one div over another. This is really simple if you know the dimensions of the div.
Solved here:
How to overlay one div over another div
So, here is my HTML:
<div class="container">
<div class="overlay"></div>
<div class="content"></div>
</div>
In my case, I don't know the exact dimensions of the "content" or "container" div. This is because I don't have control over any of the content in the div (we are making our app extensible for 3rd party developers).
See my example on jsFiddle
The overlay should cover the content entirely. Width 100% and Height 100%. However, this does not work because in my example I positioned the overlay absolutely.
One solution is to use JavaScript to get the size of the content div and then set the size of the overlay. I don't like this solution much since if image sizes are not specified, you need to wait until images are loaded and recalculate the size of the div.
Is there any way of solving this problem in CSS?

You could set the position to absolute and then set all 4 positioning values to 0px which will make the box expand. See a demo here: http://jsfiddle.net/6g6dy/
This way you dont have to worry about recalculating things if you want padding on the overlay or the container (like you would if you used actual height and width values), because its always going to be adjusted to the outer dimensions of the box.

It's not possible to do this because:
The overlay is not contained by anything to restrict it's size (since there is no height/width applied to the container).
The size of the content div can change as content loads (since it has no fixed width/height).
I solved this by using JavaScript*. Eg.
function resizeOverlay() {
$('.overlay').css({
width: $('.content').width()
height: $('.content').height()
});
}
$('.content').find('img').on('load', resizeOverlay);
*Code not tested.

Hey are you looking like this : http://tinkerbin.com/Vc4RkGgQ
CSS
.container {
position:relative;
background:blue;
color:white;
}
.content {
position:absolute;
top:0;
left:15px;
background:red;
color:yellow;
}

I do not know what you are exactly trying to do but this might work:
container must be relative: anything from static
overlay and content are absolute :move top/left in first non static parent; no flow.
Give same top/left to be on top and higher z-index for upper element.

See this demo: http://jsfiddle.net/rathoreahsan/kEsbx/
Are you trying to do as mentioned in above Demo?
CSS:
#container {
position: relative;
}
.overlay,
.content{
display:block;
position: absolute;
top: 0;
left: 0;
}
.overlay{
z-index: 10;
background: #ccc;
}

You can indeed do this without JavaScript. Your problem is that #container element has 100% width relative to the whole page. To fix this you can:
a) position it absolutely,
#container {
position: absolute;
}
b) make it float or
#container {
float: left;
}
c) make it display as table cell
#container {
display: table-cell;
}
One of the above is enough, you don't need to apply all. Also you should not position .content absolutely as this will prevent #container to have the same width/height.

If you are worried about images loading after the height is set you can go ahead and set the dimensions of the image in the containing div and use the padding-bottom hack. This way when the browsers paints over the page it knows how big the image will be before it loads.

Size of div depending on size of another div

I got a div which contains 2 other divs. They are vertically aligned. Now, the lower div is changing its size via javascript. What I want now is that the upper div is changing its size depending on the other divs size. Is there a way to do this with css style only, without using js?
UPDATE: The outer div has a fixed size.
<div>
<div> childdiv 1</div>
<div> childdiv 2</div>
</div>
UPDATE: Ok I didnt make this clear enough, the lower box is changing its height in top direction. And the upper div should then decrease its height.

You could use display:table and display:table-row to change the height of the upper div accordingly, so that the total height matches the fixed height of the container:
#outer{
display:table;
height:200px;
width:200px;
}
#inner1{
background-color:red;
display:table-row;
}
#inner2{
background-color:green;
display:table-row;
height:30px;
}
You'll find an example here: http://jsfiddle.net/bRg6m/
Is this what you want?
Added: Note that this doesn't work in IE7 or older. You'll have to use a Javascript solution if you want to support those browsers.

I'm not sure I understand your question.
I made an example here where the upper div is changing its width depending on the lower divs size. Is that what you needed?
http://jsfiddle.net/8fwXR/
HTML
<div id="wrapper">
<div id="box1"></div>
<div id="box2"></div>
</div>
CSS
#wrapper {
float: left;
}
#box1 {
background: red;
height: 100px;
width: 100%;
}
#box2 {
background: green;
height: 100px;
width: 400px;
}

Possibly this is what I meant:
document.getElementById("upperDiv").style.width = document.getElementById("bottomDiv").style.width

Really depends on your setup. The easiest way by far would be to just apply the javascript code to both DIVs at the same time.
Setting the width of the first DIV to be 100% should cause it to resize the width to match the second DIV (as the resize will force the parent DIV to increase in width too), but you will have a problem when trying to get the height to match, as with pure CSS the first DIV will have no reference to recalculate its own height to match.
Example Here
Alternatively, instead of making the second DIV resize you could resize the parent DIV and have both children set with CSS as follows:
width:100%;
height:50%;