Fix Kobweb producing bad output for browsers that don't support CSS layers

[bitspittle]

We introduced layers in Kobweb recently, except we just learned older versions of safari choke on them. They are, I'm assuming, ignoring them, which is fine, but they are also ignoring all the styles we define underneath them, which is not.

Probably the way forward is detecting if layers are supported and, if so, put normal styles there; otherwise, declare styles flat.

I have an account on BrowserStack I can use to check if things are working.

This issue is currently affecting ~5.26% of the overall market.

Other links:

• the @supports feature
• css layer fallbacks
• can I use css layers
• can I use @supports
• cascade layers polyfill npm plugin
  • Likely can't use this as our styles are declared in code, not in an actual CSS file. Also, looks like it doesn't play nice with @layers inside @media queries, which I don't think we have but that sounds like a scary gotcha.