ACTION-738 Propose text stating that proxies should generate valid markup

Hi Folks

Here's one that I'm afraid slipped off the end of my to-do list:

I'm inserting the following into the draft I am editing even as we speak:


Under 4.4 following "If the proxy alters the content then it must add a 
Warning 214 Transformation Applied HTTP Header."

If the proxy alters content then the altered content SHOULD validate 
according to an appropriate formal published grammar.

Jo