[css-extensions-1] Register custom extensions, worklets in CSS

[romainmenke]

css-extensions-1

see : #3559 (comment)

Can we register custom extensions, worklets in CSS?

@custom-supports --gap-in-flexbox url('gapInFlexboxSupport.js');

One of my concerns with registering custom extensions or worklets in JavaScript is that it forces stylesheet authors to switch context and consider a whole different set of issues and apply solutions in a different programming language.

• how can I block the CSS on this bit of JavaScript and avoid a flash of content?
• how do I write the bits of Javascript defensively so that it doesn't throw errors in older browsers?

It also paves the way for tools to provide more assistance to stylesheet authors because everything is done within a single context.