concrete syntax might change this rule via the NAMECASE NAMING declarations). Markup minimization SGML has features for reducing the number of characters required to mark up a document, which must be enabled in the SGML Declaration. SGML processors need not support every available feature, thus allowing applications to tolerate many types of inadvertent markup omissions; however, SGML systems usually are intolerant of invalid structures. XML is intolerant of syntax omissions, and does not require a DTD for checking well-formedness. OMITTAG Both start tags and end tags may be omitted from a document instance, provided: the OMITTAG feature is enabled in the SGML Declaration, the DTD indicates that the tags are permitted to be omitted, (for start tags) the element has no associated required (#REQUIRED) attributes, and the tag can be unambiguously inferred by context. For example, if OMITTAG YES is specified in the SGML Declaration (enabling the OMITTAG feature), and the DTD includes the following declarations: <!ELEMENT chapter - - (title, section+)> <!ELEMENT title o o (#PCDATA)> <!ELEMENT section - - (title, subsection+)> then this excerpt: <chapter>Introduction to SGML <section>The SGML Declaration <subsection> ... which omits two tags and two tags, would represent valid markup. Omitting tags is optional – the same excerpt could be tagged like this: <chapter><title>Introduction to SGML</title> <section><title>The SGML Declaration</title> <subsection> ... and would still represent valid markup. Note: The OMITTAG feature is unrelated to the tagging of elements whose declared content is EMPTY as defined in the DTD: <!ELEMENT image - o EMPTY> Elements defined like this have no end tag, and specifying one in the document instance would result in invalid markup. This is syntactically different from XML empty elements in this regard. SHORTREF Tags can be replaced with delimiter strings, for a terser markup, via the SHORTREF feature. This markup style is now associated with wiki markup, e.g. wherein two equals-signs (==), at the start of a line, are the "heading start-tag", and two equals signs (==) after that are the "heading end-tag". SHORTTAG SGML markup languages whose concrete syntax enables the SHORTTAG VALUE feature, do not require attribute values containing only alphanumeric characters to be enclosed within quotation marks—either double " " (LIT) or single ' ' (LITA)—so that the previous markup example could be written: <QUOTE TYPE=example> typically something like <ITALICS>this</> </QUOTE> One feature of SGML markup languages is the "presumptuous empty tagging", such that the empty end tag </> in