Enable generation of Optimized (aka Transformed) AMP pages

[westonruter]

We can speed up the serving of AMP pages by pre-rendering them and serving them in the same way they are served from an AMP cache. In short, the processes performed by the AMP Optimizer should be ported into the plugin. Something important to note here that when pre-rendering is performed, there will somewhat counter-intuitively need to be a paired mode even when a site is AMP Canonical. The reason is:

Warning: optimized AMPs will no longer be valid AMPHTML. Don't use this to optimize AMPs meant for being used by platforms such as Google Search.

So the pre-rendered AMP version of a page would would be canonical URL, and there would be an amphtml link to the same exact content without the pre-rendering logic applied. This is similar to the existing paired mode works, except here the canonical version is generated from the AMP version via pre-rendering. So it's important to note that this pre-rendering should not be offered on sites that are not using Canonical AMP, as it would not make sense to have a pre-rendered AMP version alongside the valid AMP and the non-AMP content.

As for where the pre-rendering should be performed, it should be done right after the call to AMP_Content_Sanitizer::sanitize_document() in AMP_Theme_Support::prepare_response():

https://github.com/Automattic/amp-wp/blob/1ab37c654d8c36cb1e17f1c997a6a2f00002273c/includes/class-amp-theme-support.php#L813-L815

For example:

$assets = AMP_Content_Sanitizer::sanitize_document( $dom, self::$sanitizer_classes, $args );
self::ensure_required_markup( $dom );
if ( ! is_amp_endpoint() ) {
    do_the_function_that_runs_amp_prerendering( $dom ); // 👈
}

Since is_amp_endpoint() currently always returns true if amp_is_canonical() then this will need to be amended to support the case for pre-rendering. In other words, even on a canonical-site there will be the need for separate AMP endpoint URLs. So the if ( amp_is_canonical() ) { return true; } conditional may need to be removed here. Related to #934.

👉 For more on server-side rendering, see Faster AMP on the origin: AMP + SSR = ⚡.

👉 For implementation notes, refer to AMP SSR Toolkit Implementation Guide.

[westonruter]

In AMP the SSR is also termed “transformed”. The validator has just landed with some recognition of this which we're not explicitly skipping from the generated spec: 51d81ab.

We'll potentially need to revert that and add some awareness of a transformed mode. Or potentially we'd still not care about the transformed rules since we'd apply them in a sanitizer after the whitelist runs.

[westonruter]

See also https://twitter.com/westonruter/status/1120061001443893248

We need to make sure the Transformed AMP sanitizer, along with the other sanitizers, are turned into a separate package which can be used standalone.

[westonruter]

Depends on layout=intrinsic being server-side renderable. See ampproject/amphtml#17686, ampproject/amphtml#24119, ampproject/amp-toolbox#443, ampproject/amp-toolbox#264.

[westonruter]

An important blocker for SSR (transformed intrinsic layout support) has been lifted: ampproject/amphtml#24119. For pending reference implementation of SSR intrinsic layout, see ampproject/amp-toolbox#443.

[csossi]

Any QA instructions to be added here, @schlessera ?

[schlessera]

Testing instructions:

1. Make sure the Optimizer is enabled (this is the case on the test site)
2. Go to the AMP-version of a page
3. Look at the generated markup of the page via View Source...
4. Verify: (Optimizer active) The html tag has the attribute transformed="self;v=1" (at the very beginning of the document)
5. Verify: (Serverside rendering active) There's a <style amp-runtime="" i-amphtml-version="XXX"> tag that contains the entire AMP runtime stylesheet
6. Verify: Generated page is still valid AMP (aside for dev mode when logged in)

/cc @csossi

[westonruter]

Also verify that the result is Valid AMP (aside for dev mode when logged in).

[schlessera]

Good point, updated the instructions.

[csossi]

Verified in qa: