Regular expressions are a good way to validate text fields such as names, addresses, phone numbers, and other user information. A regular expression can easily check whether a user entered something that looks like a valid phone number.
If that document includes an element from another specification, such as HTML 3.2 or HTML5, a browser will still render that part of the document. Document Type Declarations were intended for validators, not browsers.Until now, browser makers have had to individually figure out how to deal with errors.This usually involved reverse engineering whatever the most popular browser was doing— not a very productive use of their time.But if it goes too far in the other direction, the specification will enshrine tags and tables for layout because, after all, that’s what a huge number of web pages are built with.It’s a delicate balancing act that requires a pragmatic, levelheaded approach.
On the other hand, HTML5 needs to support existing content, even if most existing content is a complete mess.