Quick & dirty way to inline all of the CSS on a page into style attributes from the console.

staticCSSSnapshot.console.js

(function(){
const setProps = new WeakMap;
for (const ele of document.getElementsByTagName("*"))
	setProps.set(ele, new Set);

function parseRules(ruleSet) {
	for (const rule of ruleSet)
		if (rule.type === 1) { // STYLE_RULE
			for (const ele of document.querySelectorAll(rule.selectorText)) {
				const setObj = setProps.get(ele);
				for (const prop of rule.style)
					setObj.add(prop);
			}
		} else if (rule.type === 3 || rule.type === 4) { // import or media
			if (!rule.media.mediaText || matchMedia(rule.media.mediaText).matches) {
				if (rule.type === 4) parseRules(rule.cssRules); // MEDIA_RULE
				else try{ parseRules(rule.styleSheet.cssRules); }catch(e){} // IMPORT_RULE
			}
		} else if (rule.type === 6) { // PAGE_RULE
			// nothing to do here
		} else if (rule.type === 7) { // KEYFRAMES_RULE
			// nothing to do here
		} else if (rule.type === 12) { // SUPPORTS_RULE
			if (!rule.conditionText || CSS.supports(rule.conditionText)) {
				parseRules(rule.cssRules)
			}
		}
}

for (const sheet of document.styleSheets)
	if (!sheet.disabled && sheet.cssRules)
		try { parseRules(sheet.cssRules); } catch(e){}

for (const ele of document.getElementsByTagName("*")) {
	const compStyle = getComputedStyle(ele), liveStyle = ele.style;
	for (const cssProp of setProps.get(ele)) {
		// check if this style is really needed (to compensate for CSSReset sheets and such)
		var trueValue = compStyle[cssProp];

		liveStyle.setProperty(cssProp, "unset", "important");
		if (compStyle[cssProp] === trueValue) {

			liveStyle.removeProperty( cssProp );
		} else {

			liveStyle.setProperty(cssProp, "inherit", "important");
			if (compStyle[cssProp] === trueValue) {

				liveStyle[cssProp] = "inherit";
			} else {
				// this style property really is needed
				liveStyle[cssProp] = trueValue;
			}
		}
	}
}


for (const sheet of document.styleSheets)
	sheet.diabled = true; // disable all stylesheets because they are no longer needed.

})();