Identified HTML best practices, to create new checkers

Following is a list of identified best practices for creating web pages.

Anybody, anywhere deviating from the prescriptions below is wrong according to good reasons which are already published.

These may be good candidates for people to create new extensions in the future. (And publish them here.)

• Use lowercase doctype
  • tl;dr: DOCTYPE and doctype are both valid, but the latter compresses better.
  • Reference: https://stackoverflow.com/questions/7020961/uppercase-or-lowercase-doctype#comment71796606_7020961