Editing HTML element (section)

===Element standards===

HTML elements are defined in a series of freely available open standards issued since 1995, initially by the [[IETF]] and subsequently by the [[W3C]].

During the [[browser wars]] of the 1990s, developers of [[user agent]]s (e.g. [[web browser]]s) often developed their own elements, some of which have been adopted in later standards. Other user agents may not recognize non-standard elements, and they will be ignored, possibly causing the page to be displayed improperly.

In 1998, [[XML]] (a simplified form of SGML) introduced mechanisms to allow anyone to develop their own elements and incorporate them in [[XHTML]] documents, for use with XML-aware user agents.<ref>[[#XML10|XML 1.0]] (The ability to produce additional elements is part of the ''eXtensibility'' in the acronym.)</ref>

Subsequently, HTML 4.01 was rewritten in an [[XML]]-compatible form, [[#XHTML10|XHTML 1.0]] (''eXtensible HTML''). The elements in each are identical, and in most cases valid XHTML 1.0 documents will be valid or nearly valid HTML 4.01 documents. This article mainly focuses on real HTML, unless noted otherwise; however, it remains applicable to XHTML. See [[HTML#SGML-based versus XML-based HTML|HTML]] for a discussion of the minor differences between the two.