Allow to extract CSS into standalone files #94

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Feature request

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What problem does this feature solve?

Extract CSS for better caching.

What does the proposed API look like?

Currently:

• in prod, Saber inlines critical CSS in the initial HTML, other CSS are injected via <script> tag dynamically
• in dev, all CSS are injected dynamically

Since mini-css-extract-plugin supports hot reloading now, maybe we can use it in both dev and prod, it can be enabled with an option: build.extractCSS.

We need to ensure that CSS being used by two or more chunks are split into a vendor chunk, i.e. style-vendor.css.

How should this be implemented in your opinion?

N/A

Are you willing to work on this yourself?

PR welcome.