EDIT: This answer is now deprecated and no longer works in Chrome 64+. Leaving for historical context. In fact that bug report links back to this question for alternative solutions to using this.

Seems I managed to answer my own question after another hour of research.

It's as simple as this:

window.getMatchedCSSRules(document.getElementById("description"))

(Works in WebKit/Chrome, possibly others too)

Have a look at this library, which does what was asked for: http://www.brothercake.com/site/resources/scripts/cssutilities/

It works in all modern browsers right back to IE6, can give you rule and property collections like Firebug (in fact it's more accurate than Firebug), and can also calculate the relative or absolute specificity of any rule. The only caveat is that, although it understands static media types, it doesn't understand media-queries.

Since this question currently doesn't have a lightweight (non-library), cross-browser compatible answer, I'll try to provide one:

function css(el) {
var sheets = document.styleSheets, ret = [];
el.matches = el.matches || el.webkitMatchesSelector || el.mozMatchesSelector
|| el.msMatchesSelector || el.oMatchesSelector;
for (var i in sheets) {
var rules = sheets[i].rules || sheets[i].cssRules;
for (var r in rules) {
if (el.matches(rules[r].selectorText)) {
ret.push(rules[r].cssText);
}
}
}
return ret;
}

JSFiddle: http://jsfiddle.net/HP326/6/

Calling css(document.getElementById('elementId')) will return an array with an element for each CSS rule that matches the passed element. If you want to find out more specific information about each rule, check out the CSSRule object documentation.

Here's a version of S.B.'s answer which also returns matching rules within matching media queries. I've removed the *.rules || *.cssRules coalescence and the .matches implementation finder; add a polyfill or add those lines back in if you need them.

This version also returns the CSSStyleRule objects rather than the rule text. I think this is a little more useful, since the specifics of the rules can be more easily probed programmatically this way.

Coffee:

getMatchedCSSRules = (element) ->
sheets = document.styleSheets
matching = []


loopRules = (rules) ->
for rule in rules
if rule instanceof CSSMediaRule
if window.matchMedia(rule.conditionText).matches
loopRules rule.cssRules
else if rule instanceof CSSStyleRule
if element.matches rule.selectorText
matching.push rule
return


loopRules sheet.cssRules for sheet in sheets


return matching

JS:

function getMatchedCSSRules(element) {
var i, len, matching = [], sheets = document.styleSheets;


function loopRules(rules) {
var i, len, rule;


for (i = 0, len = rules.length; i < len; i++) {
rule = rules[i];
if (rule instanceof CSSMediaRule) {
if (window.matchMedia(rule.conditionText).matches) {
loopRules(rule.cssRules);
}
} else if (rule instanceof CSSStyleRule) {
if (element.matches(rule.selectorText)) {
matching.push(rule);
}
}
}
};


for (i = 0, len = sheets.length; i < len; i++) {
loopRules(sheets[i].cssRules);
}


return matching;
}

Short version^{12 April 2017}

Challenger appears.

var getMatchedCSSRules = (el, css = el.ownerDocument.styleSheets) =>
[].concat(...[...css].map(s => [...s.cssRules||[]])) /* 1 */
.filter(r => el.matches(r.selectorText));            /* 2 */

Line /* 1 */ builds a flat array of all rules.
Line /* 2 */ discards non-matching rules.

Based on function css(el) by @S.B. on the same page.

Example 1

var div = iframedoc.querySelector("#myelement");
var rules = getMatchedCSSRules(div, iframedoc.styleSheets);
console.log(rules[0].parentStyleSheet.ownerNode, rules[0].cssText);

Example 2

var getMatchedCSSRules = (el, css = el.ownerDocument.styleSheets) =>
[].concat(...[...css].map(s => [...s.cssRules||[]]))
.filter(r => el.matches(r.selectorText));


function Go(big,show) {
var r = getMatchedCSSRules(big);
PrintInfo:
var f = (dd,rr,ee="\n") => dd + rr.cssText.slice(0,50) + ee;
show.value += "--------------- Rules: ----------------\n";
show.value += f("Rule 1:   ", r[0]);
show.value += f("Rule 2:   ", r[1]);
show.value += f("Inline:   ", big.style);
show.value += f("Computed: ", getComputedStyle(big), "(…)\n");
show.value += "-------- Style element (HTML): --------\n";
show.value += r[0].parentStyleSheet.ownerNode.outerHTML;
}


Go(...document.querySelectorAll("#big,#show"));

.red {color: red;}
#big {font-size: 20px;}

<h3 id="big" class="red" style="margin: 0">Lorem ipsum</h3>
<textarea id="show" cols="70" rows="10"></textarea>

Shortcomings

• No media handling, no @import, @media.
• No access to styles loaded from cross-domain stylesheets.
• No sorting by selector “specificity” (order of importance).
• No styles inherited from parents.
• May not work with old or rudimentary browsers.
• Not sure how it copes with pseudo-classes and pseudo-selectors but seems to fare okay.

Maybe I will address these shortcomings one day.

Long version^{12 August 2018}

Here’s a much more comprehensive implementation taken from someone’s GitHub page (forked from this original code, via Bugzilla). Written for Gecko and IE, but is rumoured to work also with Blink.

4 May 2017: The specificity calculator has had critical bugs which I have now fixed. (I can’t notify the authors because I don’t have a GitHub account.)

12 August 2018: Recent Chrome updates seem to have decoupled object scope (this) from methods assigned to independent variables. Therefore invocation matcher(selector) has stopped working. Replacing it by matcher.call(el, selector) has solved it.

// polyfill window.getMatchedCSSRules() in FireFox 6+
if (typeof window.getMatchedCSSRules !== 'function') {
var ELEMENT_RE = /[\w-]+/g,
ID_RE = /#[\w-]+/g,
CLASS_RE = /\.[\w-]+/g,
ATTR_RE = /\[[^\]]+\]/g,
// :not() pseudo-class does not add to specificity, but its content does as if it was outside it
PSEUDO_CLASSES_RE = /\:(?!not)[\w-]+(\(.*\))?/g,
PSEUDO_ELEMENTS_RE = /\:\:?(after|before|first-letter|first-line|selection)/g;
// convert an array-like object to array
function toArray(list) {
return [].slice.call(list);
}


// handles extraction of `cssRules` as an `Array` from a stylesheet or something that behaves the same
function getSheetRules(stylesheet) {
var sheet_media = stylesheet.media && stylesheet.media.mediaText;
// if this sheet is disabled skip it
if ( stylesheet.disabled ) return [];
// if this sheet's media is specified and doesn't match the viewport then skip it
if ( sheet_media && sheet_media.length && ! window.matchMedia(sheet_media).matches ) return [];
// get the style rules of this sheet
return toArray(stylesheet.cssRules);
}


function _find(string, re) {
var matches = string.match(re);
return matches ? matches.length : 0;
}


// calculates the specificity of a given `selector`
function calculateScore(selector) {
var score = [0,0,0],
parts = selector.split(' '),
part, match;
//TODO: clean the ':not' part since the last ELEMENT_RE will pick it up
while (part = parts.shift(), typeof part == 'string') {
// find all pseudo-elements
match = _find(part, PSEUDO_ELEMENTS_RE);
score[2] += match;
// and remove them
match && (part = part.replace(PSEUDO_ELEMENTS_RE, ''));
// find all pseudo-classes
match = _find(part, PSEUDO_CLASSES_RE);
score[1] += match;
// and remove them
match && (part = part.replace(PSEUDO_CLASSES_RE, ''));
// find all attributes
match = _find(part, ATTR_RE);
score[1] += match;
// and remove them
match && (part = part.replace(ATTR_RE, ''));
// find all IDs
match = _find(part, ID_RE);
score[0] += match;
// and remove them
match && (part = part.replace(ID_RE, ''));
// find all classes
match = _find(part, CLASS_RE);
score[1] += match;
// and remove them
match && (part = part.replace(CLASS_RE, ''));
// find all elements
score[2] += _find(part, ELEMENT_RE);
}
return parseInt(score.join(''), 10);
}


// returns the heights possible specificity score an element can get from a give rule's selectorText
function getSpecificityScore(element, selector_text) {
var selectors = selector_text.split(','),
selector, score, result = 0;
while (selector = selectors.shift()) {
if (matchesSelector(element, selector)) {
score = calculateScore(selector);
result = score > result ? score : result;
}
}
return result;
}


function sortBySpecificity(element, rules) {
// comparing function that sorts CSSStyleRules according to specificity of their `selectorText`
function compareSpecificity (a, b) {
return getSpecificityScore(element, b.selectorText) - getSpecificityScore(element, a.selectorText);
}


return rules.sort(compareSpecificity);
}


// Find correct matchesSelector impl
function matchesSelector(el, selector) {
var matcher = el.matchesSelector || el.mozMatchesSelector ||
el.webkitMatchesSelector || el.oMatchesSelector || el.msMatchesSelector;
return matcher.call(el, selector);
}


//TODO: not supporting 2nd argument for selecting pseudo elements
//TODO: not supporting 3rd argument for checking author style sheets only
window.getMatchedCSSRules = function (element /*, pseudo, author_only*/) {
var style_sheets, sheet, sheet_media,
rules, rule,
result = [];
// get stylesheets and convert to a regular Array
style_sheets = toArray(window.document.styleSheets);


// assuming the browser hands us stylesheets in order of appearance
// we iterate them from the beginning to follow proper cascade order
while (sheet = style_sheets.shift()) {
// get the style rules of this sheet
rules = getSheetRules(sheet);
// loop the rules in order of appearance
while (rule = rules.shift()) {
// if this is an @import rule
if (rule.styleSheet) {
// insert the imported stylesheet's rules at the beginning of this stylesheet's rules
rules = getSheetRules(rule.styleSheet).concat(rules);
// and skip this rule
continue;
}
// if there's no stylesheet attribute BUT there IS a media attribute it's a media rule
else if (rule.media) {
// insert the contained rules of this media rule to the beginning of this stylesheet's rules
rules = getSheetRules(rule).concat(rules);
// and skip it
continue
}


// check if this element matches this rule's selector
if (matchesSelector(element, rule.selectorText)) {
// push the rule to the results set
result.push(rule);
}
}
}
// sort according to specificity
return sortBySpecificity(element, result);
};
}

Fixed bugs

• = match → += match
• return re ? re.length : 0; → return matches ? matches.length : 0;
• _matchesSelector(element, selector) → matchesSelector(element, selector)
• matcher(selector) → matcher.call(el, selector)

Ensuring IE9+, I wrote a function which calculates CSS for requested element and its children, and gives possibility to save it to a new className if needed in snippet below.

/**
* @function getElementStyles
*
* Computes all CSS for requested HTMLElement and its child nodes and applies to dummy class
*
* @param {HTMLElement} element
* @param {string} className (optional)
* @param {string} extras (optional)
* @return {string} CSS Styles
*/
function getElementStyles(element, className, addOnCSS) {
if (element.nodeType !== 1) {
return;
}
var styles = '';
var children = element.getElementsByTagName('*');
className = className || '.' + element.className.replace(/^| /g, '.');
addOnCSS = addOnCSS || '';
styles += className + '{' + (window.getComputedStyle(element, null).cssText + addOnCSS) + '}';
for (var j = 0; j < children.length; j++) {
if (children[j].className) {
var childClassName = '.' + children[j].className.replace(/^| /g, '.');
styles += ' ' + className + '>' + childClassName +
'{' + window.getComputedStyle(children[j], null).cssText + '}';
}
}
return styles;
}

Usage

getElementStyles(document.getElementByClassName('.my-class'), '.dummy-class', 'width:100%;opaity:0.5;transform:scale(1.5);');

Here is my version of getMatchedCSSRules function which support @media query.

const getMatchedCSSRules = (el) => {
let rules = [...document.styleSheets]
rules = rules.filter(({ href }) => !href)
rules = rules.map((sheet) => [...(sheet.cssRules || sheet.rules || [])].map((rule) => {
if (rule instanceof CSSStyleRule) {
return [rule]
} else if (rule instanceof CSSMediaRule && window.matchMedia(rule.conditionText)) {
return [...rule.cssRules]
}
return []
}))
rules = rules.reduce((acc, rules) => acc.concat(...rules), [])
rules = rules.filter((rule) => el.matches(rule.selectorText))
rules = rules.map(({ style }) => style)
return rules
}

I think the answer from S.B. should be the accepted one at this point but it is not exact. It is mentioned a few times that there will be some rules that may be missed. Faced with that, I decided to use document.querySelectorAll instead of element.matches. The only thing is that you would need some kind of unique identification of elements to compare it to the one you are looking for. In most cases I think that is achievable by setting its id to have a unique value. That's how you can identify the matched element being yours. If you can think of a general way to match the result of document.querySelectorAll to the element you are looking for that would essentially be a complete polyfill of getMatchedCSSRules.

I checked the performance for document.querySelectorAll since it probably is slower than element.matches but in most cases it should not be a problem. I see that it takes about 0.001 milliseconds.

I also found CSSUtilities library that advertises that it can do this but I feel its old and has not been updated in a while. Looking at its source code, it makes me think there may be cases that it misses.

As the linked question is closed as a duplicate of this, I add an answer here instead.

The unanswered part 2: "Once I found the computed style, I want to know where it comes from"

By looping over the document.styleSheets, and looking at the getComputedStyle() before and after you modify it, you can detect what stylesheet is in use. It's far from optimal, but at least it can detect if the rule you looking at is in use or not.

Here is an exemple:

<html><head>
<title>CSS Test</title>
<style id="style-a">
li {color: #333; font-size: 20px !important;}
li.bb {color: #600; font-size: 10px;}
p {margin: 5px;}
p {margin-bottom: 10px;}
</style>
<script>
window.addEventListener('DOMContentLoaded', async () => {
const selector = 'li';
// const selector = 'li.bb';
const exempleValues = {
'color': ['rgb(0, 0, 0)', 'rgb(255, 255, 255)'],
'font-size': ['10px', '12px'],
};
const delay = (t) => new Promise((k, e) => {setTimeout(k, t)});


for(const element of document.querySelectorAll(selector)) {
const elementCss = document.defaultView.getComputedStyle(element);
for(const sheet of document.styleSheets) {
for(const rule of sheet.cssRules) {
if(rule.selectorText !== selector) {
continue;
}
for(const properyName of rule.style) {
const currentValue = rule.style[properyName];
const priority = rule.style.getPropertyPriority(properyName)
if(!exempleValues[properyName]) {
console.warn('no exemple values for', properyName);
continue;
}
const exempleValue = exempleValues[properyName][exempleValues[properyName][0] === currentValue ? 1 : 0];
rule.style.setProperty(properyName, exempleValue, priority);
await delay(100);
if(exempleValue === elementCss[properyName]) {
console.log(selector, properyName, currentValue, priority || false, true, 'in use', element, sheet.ownerNode);
} else {
console.log(selector, properyName, currentValue, priority || false, false, 'overrided', element);
}
rule.style.setProperty(properyName, currentValue, priority);
await delay(100);
}
}
}
}
}, {once: true});
</script>
</head><body>
<h1>CSS Test</h1>
<p>html-file for testing css</p>
<ul>
<li>AAAA</li>
<li class="bb">BBBB</li>
<li>CCCC</li>
</ul>
</body></html>