Feature Proposal: default value of decode attribute

[ojanvafai]

Web authors want a way to set the default decode value on all images. The decode attribute is hard to use in that way as it requires control over all the code inserting images into the page and careful programming practice.

Let's add a feature policy to make it so that the default value of the decode attribute is "async".

Alternately, should it require that all images be async? Requiring has a nice advantage of being able to ensure that the third parties you embed can't override it and cause performance problems for the first party.

[Malvoz]

I've submitted a proposal for the decoding-image-default-sync policy-controlled feature in w3c/webappsec-permissions-policy#332.

Also, should this affect poster and background-images? (those questions still stand for lazy-loading https://github.com/w3c/webappsec-feature-policy/issues/311 as well.)