Perm URL with updates: http://xahlee.org/js/case_sensitivity_html5_xml_css_js.html
Case Sensitivity and Allowed Characters in Attribute Names
- HTML4, HTML5: tag names are NOT case sensitive.
- XHTML, XML: are case sensitive, and must be all lowercase.
- ID element, attributes values, in any HTML/XML are case sensitive. e.g.
Best is to always be case sensitive in your code.
Allowed chars in “id”
<div id="myID">…</div> must start with a letter (A to Z, uppper or lowercase). After that, it can be digits (0 to 9), hyphen
In practice, you can start with a unicode char in id, such as α, and the W3C validator will pass it as valid (as of 2012-03-29). (➲ Using Unicode in HTML Attributes)