wg-caching status update 2020-05-06

## What was accomplished?

- Signed Exchanges:
- AMP Packager can now be deployed on Google Cloud. [Instructions](https://github.com/ampproject/amppackager/tree/master/deploy/gcloud)
- AMP Packager now supports docker multi-stage builds reducing the image sizes by 99%. [#402](https://github.com/ampproject/amppackager/pull/402)
- Various bugfixes to [NGINX Signed Exchange Module](https://github.com/google/nginx-sxg-module).
- Google AMP Cache relevant updates:
- New origin mapping (`0-example-com-0`) live. [#26205](https://github.com/ampproject/amphtml/issues/26205)
- Limited the maximum generated image srcset to 820px in the Cache to improve LCP.
- Experiment running to have the AMP cache prefetch a heuristically determined hero image, similar to what the AMP packager does for AMP SXG documents.
- DOM Purify investigation, mentioned at last update, resulted in no planned cache changes.
- Major revamp of the AMP CSS validation engine. This was designed to support a request to have a increasingly divergent CSS specification for the AMP Email specification. Some of the addressed issues on Github include [#27507](https://github.com/ampproject/amphtml/issues/27507), [#27239](https://github.com/ampproject/amphtml/issues/27239), and [#12651](https://github.com/ampproject/amphtml/issues/12651). We now support independednt per-format css properties, vendor-prefixes, byte lengths, and URL validation.
- Progress towards open sourcing the C++ AMPHTML Validator, but work remains ongoing.
- Design agreed on with OpenJS Foundation for disentangling the AMP Runtime from Google's Cache domains. [#13](https://github.com/ampproject/wg-caching/issues/13)

## What's next?
- Bring the Web Packager towards feature parity with the AMP Packager to further support building non-AMP Signed Exchanges. For example, adding support for ACME cert renewal.
- Complete the C++ Validator Open-Sourcing.
- Add a prometheus endpoint to the AMP Packer for the next release. [#357](https://github.com/ampproject/amppackager/issues/357)
- Investigating Google Cloud Click-to-deploy marketplace inclusion for the AMP Packager.
- Wrap up the Google AMP Cache hero image experiment mentioned above.

The body is a historical status update covering AMP Packager, Web Packager, the C++ AMPHTML Validator, and a Prometheus endpoint, but it names no files, tests, or entry points. First confirm which item in “What’s next?” remains open; the issue alone does not define a specific change or completion criteria.

Written by the indexing model from the issue text.