Get matching CSS rules for a DOM element using the new CSSOM stylesheet implementation

getMatchingCSSRulesForEl.js

function getMatchingCSSRulesForEl (el) {
 return [...document.styleSheets]
    .filter(ss => { try { return ss.cssRules; } catch (e) { return false } })
    .flatMap(e => [...e.cssRules].filter(r => el.matches(r.selectorText))) 
}

Due to a weird CORS CSS implementation in Chrome the only reliable way to filter inaccessible styleSheets is to try to access them and fail.