HTML and CSS Reference
tribute names. In an XHTML document, <a> and <A> are different tags;
src and SRC are different attributes, and so are sRc and SrC ! How forgiv-
ing HTML seems now.
The XHTML DTD defines all former HTML tags and attributes using
lowercase letters. Uppercase tag or attribute names are not valid XHTML
tags or attributes.
This can be a difficult situation for any author wishing to convert existing
HTML documents into XHTML-compliant ones. Lots of web pages use up-
percase tag and attribute names, to make them stand out from the sur-
rounding lowercase content.
To become compliant, all those names must be converted to lowercase-
even the ones you used in your CSS stylesheet definitions. Fortunately,
it's easy to accomplish this kind of change with various editing tools,
and XHTML authoring systems should perform the conversion for you.
16.3.5. Quoted Attribute Values
As if all those case-sensitive attribute names weren't aggravating
enough, XHTML requires that you enclose every attribute valueeven the
numeric onesin double quotes. In HTML, you could quote anything your
heart desired, but quote marks are required only if the attribute value
included whitespace or other special characters. To be XHTML compli-
ant, every attribute must be enclosed in quotes.
is wrong in XHTML. It is correctly written as: