1. What the most popular HTTP servers actually accept
2. What the most popular HTTP clients actually send
3. What the RFC "standard" says
Personally I choose #1. I don't use the most popular HTTP clients
For example, under definition #1, I can do HTTP/1.1 pipelining with POST
The RFC "standard" often comes after the software implementation(s); most RFCs document what's already in use on the internet. As the selection of software expands, some competent software authors trying their best still often struggle to conform to RFCs. Go figure
To me, the source code of the most popular servers in use is the standard