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Pseudo element manipulation

I have this DOM
<div class="slotvideo">
<div class="posterimage"></div>
</div>
I can't modify this html code but I need to add an SVG icon. This SVG is used for a function. On click I need to reach the bottom of the page. I create this CSS
.posterimage::before {
content: "";
background-image: mysvg;
}
Now, I can't manipulate pseudo element but I can't find another solution for doing it. How could you fix this problem?

I couldn't find what do you actually want. but about manipulating pseudo-elements, it's not possible. But there is a work-around.
You can define another class name like .active change the element's class to it for controlling pseudo-element.
.posterimage {
/* anything... */
}
.posterimage.active::before {
content: "";
background-image: mysvg;
}
don't forget, psuedo-elements are inline by default, so if you want this background-image to show up, you need to make it display: block and define a set of width and height though.
and it's done. you can use that .active class to have controll of showing ::before or not by JavaScript.

Using .css() on pseudo elements with jQuery

var navIcon1 = $('.nav-toggle span, .nav-toggle span:before, .nav-toggle span:after');
if (iconPos >= audPos && iconPos < eventPos) {
navIcon.css('color', 'black');
navIcon1.css('color', 'black');
}
I am trying to change the color of bootstrap navigation. I tried this code but it is not working for me.
Can I change the CSS of pseudo elements with jQuery?

Rather than adding css in your if, add a clas to "nav-toggle" for the given position and add your color for that class

The only one of the elements you can modify using this method is .nav-toggle span, you could do:
if (iconPos >= audPos && iconPos < eventPos) {
$(".nav-toggle span").css('color', 'black');
}
As to the other ones: :before, :after etc. are pseudo elements - jQuery can't access them the way you are trying to do it.
Adding a class containing your preferred color to the elements you want to change would be a way of achieving what you want to achieve.

Change selected text color of individual element

This may not be possible, but I'd like to confirm.
You can globally change the selected text highlight color of the page with
::selection {
background: #cccccc;
}
::-moz-selection {
background: #cccccc;
}
but is it possible to change the the highlighted color for an individual element in JavaScript?
For example, if s is an element's style attribute, you can change the text and background colors using
s.color = s.backgroundColor = "#cccccc";
is there a style to change the element's highlight color?
This must be in old-fashioned JavaScript, no JQuery.
EDIT:
Also, because of performance, I need to change this to the element itself. CSS class swapping performs very poorly. The use case is that every word in a page I do not own will become it's own element. On an average page, adding a CSS class through script is taking 20-30 seconds whereas changing inline styles can be done in under 1.

Pseudo-elements and pseudo-classes aren't in the DOM, but you can use classes to achieve your result.
So add a rule like
.selectionclass::-moz-selection {
background: #cccccc;
}
to your stylesheet and add the class name selectionclass to your element.

Update CSS rule property value

I have a html element which is styled (using jquery) with a background image targeted thru its class name.
When I remove the class the background image stays - which is not what I expected or want.
test.html
<div id='log' class='tile'>HELLOWORLD</div>
test.css
.tile{
background: none;
}
test.js
$('.tile').css("background-image", "url(tile.jpg)"); // We see image
$('#log').toggleClass('tile'); // We still see image
After banging my head I think I know whats happening. The css is being applied to the element - NOT to the 'class'.
How can I target a specific css rule so that its key values can be updated?
If that makes sense.

If you wan to change the css rules of the ".tile" class, then you can do it.
There is a post that explains it very well :
function changeBackgroundImage(className, value){
var ss = document.styleSheets;
for (var i=0; i<ss.length; i++) {
var ss = document.styleSheets;
var rules = ss[i].cssRules || ss[i].rules;
for (var j=0; j<rules.length; j++) {
if (rules[j].selectorText === className) {
rules[j].style.backgroundImage = value;
}
}
}
}
You can call it like this :
changeBackgroundImage(".tile","url(tile.jpg)");

The problem is that you´re setting the background-image as an inline stlye that overrides any stylesheet rules. Toggling the class won´t have any affect.
You can either have set the background through a styleheet rule and then add a class that removes it;
#log {
background-image: url(tile.jpg);
}
#log.tile {
background: none;
}
or you could just use !important as;
.tile {
background: none !important;
}
...it might be the other way around but you get the point? :)

try removing class tile and applying new class with bg: none
in effect - when needed apply class with bg, when not needed - without

No need for jQuery in this case. You can use plain old JavaScript. Check out this tutorial:
javascriptkit.com - Changing external style sheets using the DOM

You can't change the class itself without re-writing that declaration in the stylesheet, you ARE working only with the element in the selector.
Try:
$('.tile').css("background-image","none")

$('#log').toggleClass('tile',true);

I would make the background image part of the class as a css style:
.tile {background-image: url('tile.jpg')};
and then remove the class when necessary with jquery
$('#log').removeClass('tile');

you could have two classes in your css...
.tile{
background: none;
}
.tile-w-image
{
background-image: url(tile.jpg);
}
and then with jquery just toggle the classes...
$("#log").toggleClass('tile').toggleClass('tile-w-image');
I'm sure this is just one of many ways of doing this. I hope it helps.

You are very close.
It seems like you are adding inline CSS to your element and then trying to toggle the class. You should keep CSS styling separate in most cases:
HTML:
<div id='log' class='tile'>HELLOWORLD</div>
jQuery (I imagine this should be done on click or another event):
$('#log').toggleClass('tile'); // We still see image
If the "tile" class is already written to the HTML, then toggle-ing it will remove it.
CSS:
.tile{
background-image: url(tile.jpg);
}

Change CSS of class in Javascript?

I've got a class with the display set to none I'd like to in Javascript now set it to inline I'm aware I can do this with an id with getElementById but what's the cleanest way to do it with a class?

You can do that — actually change style rules related to a class — using the styleSheets array (MDN link), but frankly you're probably better off (as changelog said) having a separate style that defines the display: none and then removing that style from elements when you want them no longer hidden.

Do you mean something like this?
var elements = document.getElementsByClassName('hidden-class');
for (var i in elements) {
if (elements.hasOwnProperty(i)) {
elements[i].className = 'show-class';
}
}
Then the CSS
.hidden-class { display: none; }
.show-class { display: inline; }

You can use getElementsByClassName in which you'll get an array of elements. However this is not implemented in older browsers. In those cases getElementsByClassName is undefined so the code has to iterate through elements and check which ones have the desired class name.
For this you should use a javascript framework such as jQuery, mootools, prototype, etc.
In jQuery it could be done with a one-liner as this:
$('.theClassName').css('display', 'inline')

you can create new style rule instead.
var cssStyle = document.createElement('style');
cssStyle.type = 'text/css';
var rules = document.createTextNode(".YOU_CLASS_NAME{display:hidden}");
cssStyle.appendChild(rules);
document.getElementsByTagName("head")[0].appendChild(cssStyle);
$("#YOUR_DOM_ID").addClass("YOUR_CLASS_NAME");

You may like to exploit/rewrite this function:
function getStyleRule(ruleClass, property, cssFile) {
for (var s = 0; s < document.styleSheets.length; s++) {
var sheet = document.styleSheets[s];
if (sheet.href.endsWith(cssFile)) {
var rules = sheet.cssRules ? sheet.cssRules : sheet.rules;
if (rules == null) return null;
for (var i = 0; i < rules.length; i++) {
if (rules[i].selectorText == ruleClass) {
return rules[i].style[property];
//or rules[i].style["border"]="2px solid red";
//or rules[i].style["boxShadow"]="4px 4px 4px -2px rgba(0,0,0,0.5)";
}
}
}
}
return null;
}
to scan all style sheets attached pass "" as third argument, otherwise something like "index.css"
ruleClass contains starting '.'
if (rules[i].selectorText && rules[i].selectorText.split(',').indexOf(property) !== -1) condition improvement found here https://stackoverflow.com/a/16966533/881375
don't forget to use javascript syntax over css properties, e.g. box-shadow vs. boxShadow

Although this is long gone, here a few remarks:
Using display: inline to make things visible again may spoil the
page flow. Some elements are displayed inline, others block etc. This
should be preserved. Hence, only define a .hidden style and remove it
to make things visible again.
How to hide: There are (at least) two ways to hide elements, one is
the above mentioned display: none which basically makes the element
behave as if it was not there, and the visibility: hidden which
renders the element invisible but keeps the space it occupies.
Depending on what you want to hide, the visibility may be a better
choice, as other elements will not move when showing/hiding an
element.
Adding/removing classes vs. manipulating CSS rules: The result is
quite different. If you manipulate the CSS rules, all elements having
a certain CSS class are affected - now and in the future, i.e. new
elements dynamically added to the DOM are also hidden, whereas when
you add/remove a class, you must make sure that newly added elements
also have the class added/removed. So, I'd say adding/removing
classes works well for static HTML, whereas manipulating CSS rules
might be a better choice for dynamically created DOM elements.

To change CLASS you need to edit document stylesheets
[...document.styleSheets[0].cssRules].find(x=> x.selectorText=='.box')
.style.display='inline';
[...document.styleSheets[0].cssRules].find(x=> x.selectorText=='.box')
.style.display='inline';
.box {
margin: 10px;
padding: 10px;
background: yellow;
display: none
}
<div class="box" >My box 1</div>
<div class="box" >My box 2</div>
<div class="box" >My box 3</div>

Best way to do it is to have a hidden class, like so:
.hidden { display: none; }
After that, there is a className attribute to every element in JavaScript. You can just manipulate that string to remove occurrences of the hidden class and add another one.
One piece of advice: Use jQuery. Makes it easier to deal with that kind of stuff, you can do it like:
$('#element_id').removeClass('hidden').addClass('something');