Preloading assets in Rollup apps

[Rich-Harris]

Currently, only client.xyz123.js is preloaded. Sapper should add preload headers for all the JS and CSS needed for the page being server-rendered

[maxmilton]

I'd like to suggest we also inject <link href=xxx rel=preload as=xxx> and <link href=xxx rel=prefetch as=xxx> into the document head for sapper export so static builds can also benefit. The headers are great since some systems can use them for HTTP/2 push but the reality is it's still early days for push.

[RobbieTheWagner]

Any chance we could get one of the PRs merged in with the fix? I would love to increase my Lighthouse score 😃

[andrewagain]

Static sites can benefit from link headers as well, depending on how they are deployed. For instance with Netlify you can create a _headers file which specifies the headers for each path: https://www.netlify.com/docs/headers-and-basic-auth/

Would be great if we could export the headers somehow, or if there is a place to hook in a plug-in to create the _headers file.

[benmccann]

The headers should be preferred over the link tags because the preload spec mentions that servers may initiate a PUSH when they see a preload header and most CDNs have standardized around this behavior to initiate a PUSH.

[benmccann]

I sent a PR that implements this for JS files when using Rollup: #1269

We could probably implement it for CSS files and maybe webfonts or other files, but it's a pretty good start and probably the most meaningful piece to get done.