On the standards track: Yes
Is Experimental: No
Is Deprecated: No
In Chrome version 1 on 2008-12-11 Note: Chrome does not defer scripts with the defer attribute when the page is served as XHTML (application/xhtml+xml), see bug 41253514 and bug 41408348
In Chrome Android version 18 on 2012-06-27 Note: Chrome Android does not defer scripts with the defer attribute when the page is served as XHTML (application/xhtml+xml), see bug 41253514 and bug 41408348
In Edge version 12 on 2015-07-29
In Firefox version 3.5 on 2009-06-30 Note: Since Firefox 3.6, the defer attribute is ignored on scripts that don't have the src attribute. However, in Firefox 3.5 even inline scripts are deferred if the defer attribute is set.
In Firefox for Android version 4 on 2011-03-29
In Internet Explorer version 10 on 2012-10-26 Note: Before version 10, Internet Explorer implemented defer by a proprietary specification. Since version 10 it conforms to the W3C specification.
In Quest Browser version 5.0 on undefined Note: Quest Browser does not defer scripts with the defer attribute when the page is served as XHTML (application/xhtml+xml), see bug 41253514 and bug 41408348
In Opera version 15 on 2013-07-02 Note: Opera does not defer scripts with the defer attribute when the page is served as XHTML (application/xhtml+xml), see bug 41253514 and bug 41408348
In Opera Android version 14 on 2013-05-21 Note: Opera Android does not defer scripts with the defer attribute when the page is served as XHTML (application/xhtml+xml), see bug 41253514 and bug 41408348
In Safari version 3 on 2007-10-26
In Safari on iOS version 2 on 2008-07-11
In Samsung Internet version 1.0 on 2013-04-27 Note: Samsung Internet does not defer scripts with the defer attribute when the page is served as XHTML (application/xhtml+xml), see bug 41253514 and bug 41408348
In WebView Android version 4.4 on 2013-12-09 Note: WebView Android does not defer scripts with the defer attribute when the page is served as XHTML (application/xhtml+xml), see bug 41253514 and bug 41408348
In WebView on iOS version 2 on 2008-07-11