Polyfills plugin should feature detect polyfills support (using webcomponents loader)

[thescientist13]

Type of Change

• New Feature Request
• Documentation / Website
• Improvement / Suggestion
• Bug
• Other (please clarify below)

Summary

Currently, polyfills are enabled by default. I think for a static site, as few dependencies should be needed, but if some are needed to support browsers, they should be opt-in, ideally through differential loading

• https://dev.to/thejohnstew/differential-serving-3dkf
• https://jasonformat.com/modern-script-loading/

Edit: we won't be implementing differential loading, instead we are using our evergreen build.

Details

1. Disable polyfills by default (this may not be possible right away, or may require coordinate work, see decouple polyfills for serialization from polyfills needed for users #193 )
2. Allow opt-in flag specifically for IE11 (es5 adapter?)
3. Support custom .browserslistrc (nice to have - new ticket?)
4. Write test cases for these use cases
5. Update README with documentation / samples of the above use cases
6. Looks like we'll need to migrate PostCSS
7. Use webcomponent-loader or everything? (but actually is bigger bundle size wise than just using bundle?)
8. Question: confirm which browsers actually need polyfills

[thescientist13]

Maybe it would be better to turn this into an RFC, more thoughts come to mind

• Differential loading
• Babel / PostCSS preset-env
• Browserslist

[thescientist13]

Looks like our Lighthouse Performance score could improve as part of this if there's a better way to handle loading of polyfills

[thescientist13]

There are about three items that are left, not really related to what true differential loading is though which is basically generating two bundles, but with preset-env we're mainly leaning towards an intelligently generated common bundle.

1. Allow opt-in flag specifically for IE11 (es5 adapter?)
2. Support custom .browserslistrc (nice to have - new ticket?)
3. Use webcomponent-loader or everything? (but actually is bigger bundle size wise than just using bundle?)

The bulk of the heavy lifting coming from polyfills though instead of the straight bundle, we should switch to using the loader, so that it can feature detect the right polyfills to load without the cost of the whole bundle.

The rest of the open items could probably be made their own (single) issue.