In 1998, the W3C (World Wide Web Consortium) decided they would not continue to evolve HTML and froze the standard at HTML 4.01. W3C instead moved forward with XML and the XHTML standard. This new standard, based on XML,  required self-closing tags and syntax rule like quoting attributes to name a few.

The XHTML was transitional, meaning you could still get away with using HTML 4.01. The hope was that this would give designers and developers the time to move to the strict XHMTL standard.  With this in place W3C began work on XHTML 2.0, which would completely change the language and was not backwards compatible.  The consortium thought this new standard was more logical and better designed and in the long run it would be worth the challenges of changing the syntax. Read More