Create documentation/guide around data-css-strict in amp4email

[patrickkettner]

re: ampproject/amphtml#32587

We should have some kind of information about what data-css-strict is, why it is required, etc

[patrickkettner]

assigning myself, @CrystalOnScript it would be great to talk about what form this should take, next time we meet

[zhangsu]

Any update on this Patrick?

[Gregable]

I'd like to ping this thread and see if we can resolve this. The main reason for the data-css-strict is that it helps developers. The rules for CSS are applied by either the validator or the webmail provider's cleanup code, but the latter is harder to reason about in advance since the email might look OK when crafting it, but arrive broken due to parts being removed.

By moving developers to data-css-strict, they will be able to see the issues that might affect their pages before sending them. I can answer more questions if there are any.