Cinnamon

Hidden text, used deceptively, can be penalized by Google. Precisely what's counted as deception is anyone's guess, but there's a risk that it's deemed a dirty SEO tactic even if it is not intended as such.

No - it uses aria-hidden to tell screen readers to ignore the synonyms.

If text is set to display: none;, Find doesn't see it at all - this much is true of all browsers. Same goes for visibility: hidden; (except for Opera, where Find matches the synonym but nothing is seen). When opacity is set to 0, most browsers match the text, but nothing is visibly highlighted (Opera is the odd man out again, highlighting the background of the matched text). When text is set to color: transparent; most browsers, including Firefox and Chrome, will highlight the area while the text stays transparent - just what we want!

That I'm aware of - some edge cases may have been missed. However, Safari does things differently. When transparent text is found, Safari will display it as black text on yellow. If the text is buried under elements with a higher z-index, it brings it to the top. Another difference: most browsers match text in the middle of a string. Safari only does so when the string is CamelCase.