How to Reset :After/:Before CSS Rules for an Element

Undo pseudo element ::before and ::after rules

http://jsfiddle.net/LQTmF/

<selector>:after{
    content:none;
}

Is there any way to reset :after/:before CSS rules for an element?

There is a DOM2 API for that matter. The correct way to do this is

document.getOverrideStyle(p, ':after').display = 'none'; // or
document.getOverrideStyle(p, ':after').cssText = 'display: none !important;';

Unfortunately, no browser has implemented it. (Webkit returns null, Firefox has no such method). It looks like CSS3 doesn't even bother talking about that anymore, maybe because the usecases are very rare.

So you're gonna have to do some id/className magic as suggested above or in the other thread

Remove ::before or ::after pseudo element CSS definition

You should just apply the pseudo-element to labels that are children of .required using the child combinator >:

ul.form li.required > label::after
{
    content: " *";
    font-weight: bold;
    color: #f66;
}

The > combinator has better IE compatibility (IE7 and up) than the ::after pseudo-element (IE9 and up). If you're able to use ::after, there is no reason not to use >. In fact, IE support for pseudo-elements is so inconsistent that IE8 recognizes CSS2 :after but not CSS3 ::after. Your code would thus not work on IE8 unless you use :after, and to support IE7 and older you need a JavaScript fix.

Reset/remove CSS styles for element only

The CSS3 keyword initial sets the CSS3 property to the initial value as defined in the spec. The initial keyword has broad browser support except for the IE and Opera Mini families.

Since IE's lack of support may cause issue here are some of the ways you can reset some CSS properties to their initial values:

.reset-this {
    animation : none;
    animation-delay : 0;
    animation-direction : normal;
    animation-duration : 0;
    animation-fill-mode : none;
    animation-iteration-count : 1;
    animation-name : none;
    animation-play-state : running;
    animation-timing-function : ease;
    backface-visibility : visible;
    background : 0;
    background-attachment : scroll;
    background-clip : border-box;
    background-color : transparent;
    background-image : none;
    background-origin : padding-box;
    background-position : 0 0;
    background-position-x : 0;
    background-position-y : 0;
    background-repeat : repeat;
    background-size : auto auto;
    border : 0;
    border-style : none;
    border-width : medium;
    border-color : inherit;
    border-bottom : 0;
    border-bottom-color : inherit;
    border-bottom-left-radius : 0;
    border-bottom-right-radius : 0;
    border-bottom-style : none;
    border-bottom-width : medium;
    border-collapse : separate;
    border-image : none;
    border-left : 0;
    border-left-color : inherit;
    border-left-style : none;
    border-left-width : medium;
    border-radius : 0;
    border-right : 0;
    border-right-color : inherit;
    border-right-style : none;
    border-right-width : medium;
    border-spacing : 0;
    border-top : 0;
    border-top-color : inherit;
    border-top-left-radius : 0;
    border-top-right-radius : 0;
    border-top-style : none;
    border-top-width : medium;
    bottom : auto;
    box-shadow : none;
    box-sizing : content-box;
    caption-side : top;
    clear : none;
    clip : auto;
    color : inherit;
    columns : auto;
    column-count : auto;
    column-fill : balance;
    column-gap : normal;
    column-rule : medium none currentColor;
    column-rule-color : currentColor;
    column-rule-style : none;
    column-rule-width : none;
    column-span : 1;
    column-width : auto;
    content : normal;
    counter-increment : none;
    counter-reset : none;
    cursor : auto;
    direction : ltr;
    display : inline;
    empty-cells : show;
    float : none;
    font : normal;
    font-family : inherit;
    font-size : medium;
    font-style : normal;
    font-variant : normal;
    font-weight : normal;
    height : auto;
    hyphens : none;
    left : auto;
    letter-spacing : normal;
    line-height : normal;
    list-style : none;
    list-style-image : none;
    list-style-position : outside;
    list-style-type : disc;
    margin : 0;
    margin-bottom : 0;
    margin-left : 0;
    margin-right : 0;
    margin-top : 0;
    max-height : none;
    max-width : none;
    min-height : 0;
    min-width : 0;
    opacity : 1;
    orphans : 0;
    outline : 0;
    outline-color : invert;
    outline-style : none;
    outline-width : medium;
    overflow : visible;
    overflow-x : visible;
    overflow-y : visible;
    padding : 0;
    padding-bottom : 0;
    padding-left : 0;
    padding-right : 0;
    padding-top : 0;
    page-break-after : auto;
    page-break-before : auto;
    page-break-inside : auto;
    perspective : none;
    perspective-origin : 50% 50%;
    position : static;
    /* May need to alter quotes for different locales (e.g fr) */
    quotes : '\201C' '\201D' '\2018' '\2019';
    right : auto;
    tab-size : 8;
    table-layout : auto;
    text-align : inherit;
    text-align-last : auto;
    text-decoration : none;
    text-decoration-color : inherit;
    text-decoration-line : none;
    text-decoration-style : solid;
    text-indent : 0;
    text-shadow : none;
    text-transform : none;
    top : auto;
    transform : none;
    transform-style : flat;
    transition : none;
    transition-delay : 0s;
    transition-duration : 0s;
    transition-property : none;
    transition-timing-function : ease;
    unicode-bidi : normal;
    vertical-align : baseline;
    visibility : visible;
    white-space : normal;
    widows : 0;
    width : auto;
    word-spacing : normal;
    z-index : auto;
    /* basic modern patch */
    all: initial;
    all: unset;
}

/* basic modern patch */

#reset-this-root {
    all: initial;
    * {
        all: unset;
    }
}

Relevent github repo with a december 2017 more exaustive list
Related
Related from MDN
Related W3C specs

As mentioned in a comment by @user566245 :

this is correct in principle, but individual mileage may vary. For
example certain elements like textarea by default have a border,
applying this reset will render those textarea's border less.

JAVASCRIPT ?

Nobody thought about other than css to reset css? Yes?

There is that snip fully relevant : https://stackoverflow.com/a/14791113/845310

getElementsByTagName("*") will return all elements from DOM. Then you
may set styles for each element in the collection:

answered Feb 9 '13 at 20:15 by VisioN

var allElements = document.getElementsByTagName("*");
for (var i = 0, len = allElements.length; i < len; i++) {
    var element = allElements[i];
    // element.style.border = ...
}

With all this said; i don't think a css reset is something feasable unless we end up with only one web browser .. if the 'default' is set by browser in the end.

For comparison, here is Firefox 40.0 values list for a
<blockquote style="all: unset;font-style: oblique"> where font-style: oblique triggers DOM operation.

align-content: unset;
align-items: unset;
align-self: unset;
animation: unset;
appearance: unset;
backface-visibility: unset;
background-blend-mode: unset;
background: unset;
binding: unset;
block-size: unset;
border-block-end: unset;
border-block-start: unset;
border-collapse: unset;
border-inline-end: unset;
border-inline-start: unset;
border-radius: unset;
border-spacing: unset;
border: unset;
bottom: unset;
box-align: unset;
box-decoration-break: unset;
box-direction: unset;
box-flex: unset;
box-ordinal-group: unset;
box-orient: unset;
box-pack: unset;
box-shadow: unset;
box-sizing: unset;
caption-side: unset;
clear: unset;
clip-path: unset;
clip-rule: unset;
clip: unset;
color-adjust: unset;
color-interpolation-filters: unset;
color-interpolation: unset;
color: unset;
column-fill: unset;
column-gap: unset;
column-rule: unset;
columns: unset;
content: unset;
control-character-visibility: unset;
counter-increment: unset;
counter-reset: unset;
cursor: unset;
display: unset;
dominant-baseline: unset;
empty-cells: unset;
fill-opacity: unset;
fill-rule: unset;
fill: unset;
filter: unset;
flex-flow: unset;
flex: unset;
float-edge: unset;
float: unset;
flood-color: unset;
flood-opacity: unset;
font-family: unset;
font-feature-settings: unset;
font-kerning: unset;
font-language-override: unset;
font-size-adjust: unset;
font-size: unset;
font-stretch: unset;
font-style: oblique;
font-synthesis: unset;
font-variant: unset;
font-weight: unset;
font: ;
force-broken-image-icon: unset;
height: unset;
hyphens: unset;
image-orientation: unset;
image-region: unset;
image-rendering: unset;
ime-mode: unset;
inline-size: unset;
isolation: unset;
justify-content: unset;
justify-items: unset;
justify-self: unset;
left: unset;
letter-spacing: unset;
lighting-color: unset;
line-height: unset;
list-style: unset;
margin-block-end: unset;
margin-block-start: unset;
margin-inline-end: unset;
margin-inline-start: unset;
margin: unset;
marker-offset: unset;
marker: unset;
mask-type: unset;
mask: unset;
max-block-size: unset;
max-height: unset;
max-inline-size: unset;
max-width: unset;
min-block-size: unset;
min-height: unset;
min-inline-size: unset;
min-width: unset;
mix-blend-mode: unset;
object-fit: unset;
object-position: unset;
offset-block-end: unset;
offset-block-start: unset;
offset-inline-end: unset;
offset-inline-start: unset;
opacity: unset;
order: unset;
orient: unset;
outline-offset: unset;
outline-radius: unset;
outline: unset;
overflow: unset;
padding-block-end: unset;
padding-block-start: unset;
padding-inline-end: unset;
padding-inline-start: unset;
padding: unset;
page-break-after: unset;
page-break-before: unset;
page-break-inside: unset;
paint-order: unset;
perspective-origin: unset;
perspective: unset;
pointer-events: unset;
position: unset;
quotes: unset;
resize: unset;
right: unset;
ruby-align: unset;
ruby-position: unset;
scroll-behavior: unset;
scroll-snap-coordinate: unset;
scroll-snap-destination: unset;
scroll-snap-points-x: unset;
scroll-snap-points-y: unset;
scroll-snap-type: unset;
shape-rendering: unset;
stack-sizing: unset;
stop-color: unset;
stop-opacity: unset;
stroke-dasharray: unset;
stroke-dashoffset: unset;
stroke-linecap: unset;
stroke-linejoin: unset;
stroke-miterlimit: unset;
stroke-opacity: unset;
stroke-width: unset;
stroke: unset;
tab-size: unset;
table-layout: unset;
text-align-last: unset;
text-align: unset;
text-anchor: unset;
text-combine-upright: unset;
text-decoration: unset;
text-emphasis-position: unset;
text-emphasis: unset;
text-indent: unset;
text-orientation: unset;
text-overflow: unset;
text-rendering: unset;
text-shadow: unset;
text-size-adjust: unset;
text-transform: unset;
top: unset;
transform-origin: unset;
transform-style: unset;
transform: unset;
transition: unset;
user-focus: unset;
user-input: unset;
user-modify: unset;
user-select: unset;
vector-effect: unset;
vertical-align: unset;
visibility: unset;
white-space: unset;
width: unset;
will-change: unset;
window-dragging: unset;
word-break: unset;
word-spacing: unset;
word-wrap: unset;
writing-mode: unset;
z-index: unset;

Reset pseudo class changes in css

Increase the specificity of the last selector in order to obtain the needed result.

Here is an example where all the selectors have the same specificity (class + pseudo-class) and the last one will win:

li:not(:first-child) { /* Hide all li elements other than the first one */  display: none;}
li:first-child { /* Set list item 1 to be red */  color: red;}
/* Undo all changes above */li:nth-child(n) { /* Make all li elements have this property ()*/  display: block;  color: black;}

<ul>  <li>Item 1</li>  <li>Item 2</li>  <li>Item 3</li></ul>

Selecting and manipulating CSS pseudo-elements such as ::before and ::after using javascript (or jQuery)

You could also pass the content to the pseudo element with a data attribute and then use jQuery to manipulate that:

In HTML:

<span>foo</span>

In jQuery:

$('span').hover(function(){
    $(this).attr('data-content','bar');
});

In CSS:

span:after {
    content: attr(data-content) ' any other text you may want';
}

If you want to prevent the 'other text' from showing up, you could combine this with seucolega's solution like this:

In HTML:

<span>foo</span>

In jQuery:

$('span').hover(function(){
    $(this).addClass('change').attr('data-content','bar');
});

In CSS:

span.change:after {
    content: attr(data-content) ' any other text you may want';
}

How can I inspect and tweak :before and :after pseudo-elements in-browser?

In Chrome's Dev tools, the styles of a pseudo-element are visible in the panel:

Sample Image

Otherwise, you can also input the following line in the JavaScript console, and inspect the returned CSSStyleDeclaration object:

getComputedStyle(document.querySelector('html > body'), ':before');

window.getComputedStyle
document.querySelector

Add line break to ::after or ::before pseudo-element content

The content property states:

Authors may include newlines in the generated content by writing the "\A" escape sequence in one of the strings after the 'content' property. This inserted line break is still subject to the 'white-space' property. See "Strings" and "Characters and case" for more information on the "\A" escape sequence.

So you can use:

#headerAgentInfoDetailsPhone:after {
  content:"Office: XXXXX \A Mobile: YYYYY ";
  white-space: pre; /* or pre-wrap */
}

http://jsfiddle.net/XkNxs/

When escaping arbitrary strings, however, it's advisable to use \00000a instead of \A, because any number or [a-f] character followed by the new line may give unpredictable results:

function addTextToStyle(id, text) {
  return `#${id}::after { content: "${text.replace(/"/g, '\\"').replace(/\n/g, '\\00000a')} }"`;
}

Cannot change the content of visited :before pseudo-elements

The allowed (= not ignored) CSS properties of visited links are color, background-color, border-*-color, outline-color and, column-rule-color (more under certain circumstances).

This is to prevent history stealing attacks. See this article for further details.

So you can, technically, set a :before pseudo class for :visited links, but it will be ignored and appears as if the links are not visited. This is not a bug, it's a feature ;)