1. Key points to remember

[11] For Perl programming, some prefer the look of '$item' to \"$item\". Single and double quoting of attributes are valid in SGML, and XML (though you cannot combine the two in a single element attribute). HTML 4 is SGML-based, while XHTML 1 is XML-based. The HTML 4.01 spec.[o] explicitly states both are valid. In contrast, the XHTML 1.0 spec. just reads[o]attribute values must be quoted”. XHTML compliant documents are, however, by definition valid XML: The XML 1.0 spec. implicitly notes[o] either single or double quotation marks may be used to delimit (quote) attributes.