HTML and CSS Reference
Appendix B. XHTML Syntax Overview
XHTML (eXtensible HyperText Markup Language) is a reformulation of HTML according to
thestricter syntaxrulesofXML(eXtensible MarkupLanguage).HTML5canbewrittenusing
XHTML syntax (formally called the XML Serialization of HTML5). This section describes
the ways in which XHTML syntax differs from HTML.
Because XHTML is an XML language, its syntax is stricter and differs from HTML in these
▪ All element names and attributes must be lowercase. For example, <a
▪ All elements must be terminated—that is, they must include an end tag. For example,
▪ Empty elements must be terminated as well. This is done by including a slash at the end
of the tag. A space is commonly added before the slash for backward compatibility with
older browsers—for example, <hr /> , <img /> , and <meta /> .
▪ All attribute values must be contained in quotation marks (either single or double). For
example, <td colspan="2"> .
▪ All attribute values must be explicit and may not be minimized to one word, as is permit-
ted in HTML—for example, checked="checked" .
▪ Nesting restrictions are more strictly enforced. These restrictions are explicitly stated:
— An a element cannot contain another a element.
— The pre element cannot contain img , object , applet , big , small , sub , sup , font , or
— The form element may not contain other form elements.