Compiler render_ssr does not respect css option

[JohnGurin]

svelte 3.12.1
https://svelte.dev/examples#styling

svelte/src/compiler/compile/render_ssr/index.ts

Lines 24 to 26 in 07a4eb4

	const css = options.customElement ?
	{ code: null, map: null } :
	component.stylesheet.render(options.filename, true);

A possible fix:

const css = options.css === false || options.customElement ?
	{ code: null, map: null } :
	component.stylesheet.render(options.filename, true);

[Conduitry]

The documentation for the css option reads:

If true, styles will be included in the JavaScript class and injected at runtime. It's recommended that you set this to false and use the CSS that is statically generated, as it will result in smaller JavaScript bundles and better performance.

Perhaps this could be clarified to say that it only applies to the client-side code. But it has nothing to do with whether the .css value in the output from the compiler is present or not. You can use it or not, depending on whether you want it. The .css value is there in the output in both generate: 'browser' and generate: 'ssr' mode, whether or not you specify css: false, and this is intentional.

[JohnGurin]

...But` it has nothing to do with whether the .css value in the output from the compiler is present or not...

It is not about a compiler's output. The compiler always emits both js and css.
It is about compiled code, which will be run on a server.

I don't see any reasons why

const css = {code: "...",  map: "..."} // <----- this
const Component = create_ssr_component(($$result, ...) => {
        $$result.css.add(css); // <---- and this
        return '...';
});

is in js.code compiler output when css:false

const App = require('./compiled-app')
const { html, css, head } = App.render({ answer: 42 }); // css should be null

[TehShrike]

Playing around in the REPL, it looks like when you're generating SSR, the only effect that turning on customElement has is to prevent the CSS from being included in the JS output.

I'm not sure why the customElement toggle would have that effect for the SSR output?

[Rich-Harris]

SSR doesn't really work with custom elements full-stop. (Part of the reason I never use them, and recommend that other people don't use them either.) If there's anything that needs to change there, it's that this...

svelte.compile(text, {
  customElement: true,
  generate: 'ssr'
});

...should result in an error being thrown.

To be clear, the css: true option is designed to only control whether the add_css function is created in a client-side component to inject styles with JavaScript, which means that the resulting module is portable (i.e. you can freely import it anywhere without worrying about having to wire up static CSS files). It's not designed to have any effect in SSR mode. Perhaps it should have been called injectCss or something instead.

Is the concern here that extra work is being done unnecessarily, or is the presence of a non-null rendered.css value actively harmful somehow?

[JohnGurin]

If I build bundles for DOM and SSR from the same source, I extract CSS as an asset and inject it as a link in HTML.
Then I use SSR compiled app and I don't use CSS from app.render. SSR should be fast, what you say here is "let it be there for no reason"

Is the concern here that extra work is being done unnecessarily

Yep

[antony]

Closing for now unless this is considered an issue that requires fixing.

[jimafisk]

My use case is the same as @JohnGurin:

I build bundles for DOM and SSR from the same source, I extract CSS as an asset and inject it as a link in HTML

For my project, I also unfortunately have to do some regex parsing of the SSR output before using it to create static HTML because I'm trying to do everything in v8. The const css = {code: "...", map: "..."} output makes this process harder and isn't being used at all.

Should this just be omitted in SSR by default? It's possible I'm reading @Rich-Harris's comment wrong:

It's not designed to have any effect in SSR mode.

At the very least it would be great if passing css: false worked when using generate: 'ssr'.