HTML Minification RFC

proposals/html-minification.md

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+- Start Date: 2023-05-08
+- Reference Issues: https://github.com/withastro/roadmap/issues/537
+- Implementation PR: https://github.com/withastro/astro/pull/6706
+
+# Summary
+
+Provide a config value to enable HTML minification.
+
+# Example
+
+In the Astro config:
+
+```js
+import { defineConfig } from 'astro/config';
+
+export default defineConfig({
+  compressHTML: true
+})
+```
+
+This will enable HTML minfication for all pages.
+
+# Background & Motivation
+
+This is an often requested feature as minified HTML is a performance improvement some site measuring tools recommend.
+
+However there are some difficulties that have prevented it from being implemented sooner:
+
+- HTML minification can cause server/client hydration mismatches
+- Streaming uses newlines to delineate when to flush buffered text in a response
+- Frameworks can use HTML comments as expression markers, which can cause issues if these are removed by a minifier
+
+HTML minification is enabled in the compiler via the `compress` option, so not a lot of work is needed to enable this feature.
+
+# Goals
+
+- Safely (non-destructively) collapse whitespace during `.astro` file compilation.
+- Allow users to opt-in to minification, keeping it disabled to prevent unexpected output.
+
+# Non-Goals
+
+- Minifying non-`.astro` components. This can be done with middleware.
+- Minifying the full page. This would prevent streaming.
+
+# Detailed Design
+
+In `@astrojs/compiler` 1.0 support for the `compress` option was added. When enabled this option will remove whitespace when a `.astro` component is compiled to JavaScript.
+
+In Astro we'll add a `compressHTML` option that defaults to `false`. This option is passed directly to the `compress` option in the compiler.
+
+This is a top-level configuration value so that you get the same result in development and production modes.
+
+# Testing Strategy
+
+Fixture based tests are best here since this feature touches the config through rendering. Tests will check the output and ensure that they are 
+
+How will this feature's implementation be tested? Explain if this can be tested with
+unit tests or integration tests or something else. If relevant, explain the test
+cases that will be added to cover all of the ways this feature might be used.
+
+# Drawbacks
+
+- This approach does not compress non-`.astro` component usage.
+- There is some overlap with middleware. A middleware would be able to compress each chunk. The downside is that a user would need to import and use this middleware.
+
+# Alternatives
+
+- This could be implemented as a middleware function that a user could import and use.
+  - **Downside**: Middleware is still experimental.
+  - **Downside**: This is a lot of code just to enable a feature. User has to learn middleware where they otherwise might not be using it.
+  - **Downside**: Compression is already enabled in the compiler, so this is a small change to allow it to happen.
+
+# Adoption strategy
+
+- This is a small change so no experimental release is thought to be needed.
+- Preview release will occur to allow user testing.
+- Full release with a merged RFC in the next `minor`.
+- Documented via the config reference page.