Add plain HTML/CSS version with wider browser support

[Krinkle]

Fixes #1.

I considered using all: revert but realized that it doesn't work yet in browsers besides Firefox.

I also considered using all: initial or all: unset but it quickly became a mess. I'm willing to give that another go though if you prefer. That would mean that in modern browsers it has more or less the same guruantees as Shadow DOM, but woul degrade in legacy browsers with the same, slightly different, or very differnet rendering depending on what the site-local conflicts would be. I preferred the approach I submitted here as it makes it the same in all browsers and thus doesn't require the developer to test it out on the legacy browsers they want to support. Any conflicts would either be minor and acceptable, or dealt with by them upfront.

Thanks for sharing this repo ❤️