Web Design in a Nutshell, 3rd Edition

All-Lowercase Element and Attribute Names

In XML, all elements and attribute names are case-sensitive, which means that <img>, <Img>, and <IMG> are parsed as different elements. In the reformulation of HTML into XHTML, all elements were interpreted to be lowercase. When writing XHTML documents (and their associated style sheets), be sure that all tags and attribute names are written in lowercase. Attribute values are not required to be case-sensitive.

Note

If you want to convert the upper- and mixed-case tags in an existing HTML file to well-formed, all-lowercase tags, try the HTML Tidy utility (http://tidy.sourceforge.net/) or Barebones Software BBEdit (Macintosh only, http://www.bbedit.com), which can automate the process.