Vue SSR render initial css chunks correctly

[clarkdo]

What problem does this feature solve?

Now webpack4 mini-css-extract-plugin supports css async-loading and splitted chunks, it also has been supported in vue-loader-15

But when we use it in ssr, it will still get all css files at ssr rendering.

Through my investigation, current vue-ssr webpack plugin will not generate corresponding css into initial and async inside vue-ssr-client-manifest.json.
https://github.com/vuejs/vue/blob/dev/src/server/webpack-plugin/client.js#L19-L26

And also renderer renderStyles will render all css which includes splitted async css chunks.
https://github.com/vuejs/vue/blob/dev/src/server/template-renderer/index.js#L112

What does the proposed API look like?

css - initial chunks are in initial
css - spiltted chunks are in async
context.renderStyles() return all initial chunks
mini-css-extract-plugin will auto load async chunks