Document.xmlVersion

Obsolete since Gecko 10.0 (Firefox 10.0 / Thunderbird 10.0 / SeaMonkey 2.7)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

Returns the version number as specified in the XML declaration (e.g., <?xml version="1.0"?>) or "1.0" if the declaration is absent.

This attribute was never really useful, since it always returned 1.0, and has been removed in DOM Level 4. As such, Firefox 10 no longer implements it. Its primary use in the past was to detect whether or not the document was being rendered as XML rather than HTML. To detect this, you can create an element with its name in lower case, then check to see if it gets converted into all upper case (in which case the document is in the non-XML HTML mode):

if (document.createElement("foo").tagName == "FOO") {
  /* Document is not XML */
}

Specifications

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
xmlVersion
DeprecatedNon-standard
Chrome Full support 1Edge Full support 12Firefox No support 1 — 10IE Full support 9Opera ? Safari Full support 3WebView Android Full support 1Chrome Android Full support 18Firefox Android No support 4 — 10Opera Android ? Safari iOS Full support 1Samsung Internet Android Full support 1.0

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
Non-standard. Expect poor cross-browser support.
Non-standard. Expect poor cross-browser support.
Deprecated. Not for use in new websites.
Deprecated. Not for use in new websites.