[1] [DFN[[RUBY[空要素][くう (から) ようそ]@en[empty element]]]]は、
[[HTML]] や [[SGML]] や [[XML]] において「[[空]]」である[[要素]]を指します。
意味は何種類かあります。

* 内容 (モデル) が空の要素 = 空要素 (HTML)

[3] [[HTML]] では、[[要素]]の[[内容]] (あるいは[[内容モデル]]) が[[空]]である[[要素]]のことを
「[[空要素]]」といいます。

[4] [[HTML]] では[[空]]といっても本当に何も無い ([CODE(DOMa)@en[[[childNodes]]]]
の [CODE(DOMa)@en[[[length]]]] が0である) 状態''だけではありません''。
[[要素間空白]]、[[注釈節点]]、[[処理指令節点]]は[[空]]かどうかの判断に当たっては無視されます。

;; [[Web Applications 1.0]] <http://www.whatwg.org/specs/web-apps/current-work/complete.html#content-models>

* メモ

[2]

>Use of empty elements within XML instance documents is a source of controversy for a variety of reasons. An empty element does not simply represent data that is missing. It may express data that is not applicable for some reason, trigger the expression of an attribute, denote all possible values instead of just one, mark the end of a series of data, or appear as a result of an error in XML file generation. Conversely, missing data elements can also have meaning — data not provided by a trading partner. In information exchange environments, different trading partners may allow, require, or ban empty elements. UBL has determined that empty elements do not provide the level of assurance necessary for business information exchanges and therefore will not be used.

;; [CITE[Universal Business Language v2.0]] ([TIME[2006-12-12 21:00:00 +09:00]] 版) <http://docs.oasis-open.org/ubl/os-UBL-2.0/UBL-2.0.html#d0e3645>