Constructable Stylesheet Polyfill — 330 bytes / 235 bytes gzipped — https://wicg.github.io/construct-stylesheets/

ConstructableStylesheetPolyfill.js

try {
	new CSSStyleSheet()
} catch {
	CSSStyleSheet = ((
		stylePtt = CSSStyleSheet.prototype,
		styleDoc = new Document(),
		styleDom = styleDoc.appendChild(document.createElement('style')),
		sheet
	) => Object.assign(stylePtt.constructor = function CSSStyleSheet() {
			if (!new.target) {
				throw new TypeError("Constructor requires 'new' operator")
			}

			sheet = styleDom.sheet

			styleDoc.removeChild(styleDom)
			styleDoc.appendChild(styleDom)

			return sheet
		}.bind(), { prototype: stylePtt })
	)()
}

ConstructableStylesheetPolyfill.js.min.js

try{new CSSStyleSheet}catch{CSSStyleSheet=((e=CSSStyleSheet.prototype,t=new Document,r=t.appendChild(document.createElement("style")),n)=>Object.assign(e.constructor=function(){if(!new.target)throw new TypeError("Constructor requires 'new' operator");return n=r.sheet,t.removeChild(r),t.appendChild(r),n}.bind(),{prototype:e}))()}