Most administrators are lazy. If you write software for thousands or millions of users, sane defaults are a must. I find it useful to keep Saint-Exupéry in mind: "It seems that perfection is attained not when there is nothing more to add, but when there is nothing more to remove."

Let's look at the first lines of WordPress's default template:

  <!DOCTYPE html>
  <html xmlns="http://www.w3.org/1999/xhtml" dir="ltr" lang="en-US">
  <head profile="http://gmpg.org/xfn/11">

Why bother with xmlns if it's not well-formed XML anyway?

Why dir and lang if you don't really know what language the content is in? We now have the funny situation that most non-English pages wrongly declare lang="en-US" because W3C evangelists told template authors that lang is really important, rendering the attribute completely useless.

What the heck is profile="http://gmpg.org/xfn/11" supposed to do?