Ever wanted to reduce all the CSS in a test-case to the minimmum? :)

reduce-css.js

var USELESS_PROPERTIES = [];

function processContainer(container) {
  if (container instanceof CSSSupportsRule)
    if (!CSS.supports(container.conditionText))
      return false;
  if (container instanceof CSSMediaRule)
    if (!matchMedia(container.conditionText).matches)
      return false;
  if (container.media && container.media.mediaText)
    if (!matchMedia(container.media.mediaText).matches)
      return false;
  for (let rules = container.cssRules, i = 0; i < rules.length; ++i) {
    let rule = rules[i];
    if (!processRule(rule)) {
      container.deleteRule(i);
      i--;
    }
  }
  return !!container.cssRules.length;
}

function processRule(rule) {
  if (rule instanceof CSSGroupingRule)
    return processContainer(rule);
  if (!(rule instanceof CSSStyleRule))
    return false;
  let selector = rule.selectorText.replace(/::before/g, "").replace(/::after/g, "");
  if (selector.includes("form") || selector.includes("dropdown"))
    return true;
  if (!document.querySelector(selector))
    return false;
  let style = rule.style;
  for (let i = 0; i < style.length; ++i) {
    let property = style[i];
    for (const prop of USELESS_PROPERTIES) {
      if (property.indexOf(prop) != -1) {
        style.removeProperty(property);
        i--;
      }
    }
  }
  return !!style.length;
}

for (let sheet of document.styleSheets)
  if (!processContainer(sheet))
    if (sheet.ownerNode)
      sheet.ownerNode.remove();