I am creating a training session for a course I am studying, and am needing to add an explanation of why capitals shouldn’t be used in XHTML. Could someone please give me good explanation? Thanks in advance.
From XHTML 1.0: The Extensible HyperText Markup Language (Second Edition)
XHTML documents must use lower case for all HTML element and attribute names. This difference is necessary because XML is case-sensitive e.g. <li> and <LI> are different tags.
Is “because the rules of XHTML say they’re not allowed” good enough? 
A wide-angle disintegration beam will hit you squarely in the chest if you don’t… Like was mentioned in XML the elements and attributes are lower-case. Technically XHTML is an application of XML, unlike HTML 4.01, which is not case sensitive in that manner.