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    Site Compatibility for Firefox 19

    Firefox 19 was released on . While it has been developed to maintain compatibility as much as possible, the new version includes some changes affecting backward compatibility aimed at improving interoperability with the other browsers or following the latest Web standards. Here's the list of such changes — Hope this helps whenever you test your sites or applications.

    This article only explains the changes that may affect backward compatibility for Websites. For the other new features and changes, please read the following documents:

    Follow @FxSiteCompat on Twitter for further updates.


    The flexbox has been unprefixed

    The CSS3 flexible boxes (flexbox) implementation has been prefixed. From now on, use the related properties and keywords without moz prefix. Note that the flexbox is still disabled by default in Firefox 19. If you'd like to test the feature, open about:config and change the value of layout.css.flexbox.enabled to true.

    -moz-initial has been unprefixed

    The -moz-initial keyword has been unprefixed. While -moz-initial will remain in the meantime as an alias of initial, it will be removed at some time, so use the unprefixed keyword instead.

    The :-moz-placeholder pseudo-class has been replaced with the pseudo-element

    The :-moz-placeholder pseudo-class that matches form elements with the placeholder attribute has been removed, and the ::-moz-placeholder pseudo-element has been added instead. The implementation of WebKit has been a pseudo-element, and this change is a part of the standardization effort.

    Key frame rule declarations with !important will be ignored

    Following the latest CSS3 animations spec, key frame rule declarations with the !important keyword are now be ignored and parse errors will be returned.

    Regression: background images specified with -moz-element are not updated

    CSS background images specified with the -moz-element function are not updated when the background-image of the source element is dynamically changed. This is a regression since Firefox 19, which has been fixed with Firefox 26.

    FYI: Preferences to disable prefixed properties have been added

    While this is not a change affecting site compatibility, it's worth mentioning because this has been developed as part of efforts to keep compatibility. Preferences to disable some major prefixed properties have been added: layout.css.prefixes.border-image, layout.css.prefixes.transforms, layout.css.prefixes.transitions and layout.css.prefixes.animations. Web developers can disable those preferences (change the values to false) to test whether style rules are applied as intended even after those prefixed implementations are removed. See the David Baron's blog post for details.


    Element.getElementsBy* now returns HTMLCollection

    The type of element lists that can be retrieved with the getElementsByTagName, getElementsByTagNameNS and getElementsByClassName methods have been changed from NodeList, defined in the DOM3 Core spec, to HTMLCollection, defined in the DOM4 spec draft.

    hasFeature/isSupported methods now always return true

    The document.implementation.hasFeature and Node.isSupported methods have been changed to always return true. The spec has been changed because those APIs were considered useless. However the SVG features are exception; those methods continue to return the support statuses.

    createElement(null) no longer throws exceptions

    Previously the document.createElement method has thrown exception INVALID_CHARACTER_ERR if the argument was null. The method now returns the HTMLUnknownElement object because the argument should be treated as a string and considered to be the same code as document.createElement("null").

    The implementation of document.referrer has been updated for the latest spec

    When the URL of a nested inline frame (<iframe>) or a grandchild window is programmatically changed from the parent window, the value of the document.referrer property now points the URL of the parent window where the script is written instead of the child window that refers directly. This is due to a change of the spec and leads to the same behavior as Internet Explorer and Opera. WebKit to follow.

    File.lastModifiedDate now returns the current date if the modified date is unknown

    Following the latest File API spec, the lastModifiedDate property of a File object now returns the current date if the file's last modified date is unknown. Previously it returns null in such case.

    The Encoding API has been updated for the latest spec

    Following a change of the Encoding API spec, the implementation of TextEncoder and TextDecoder has been updated.

    XForms support has been removed

    The XML Events implementation has been removed. The development of the Mozilla XForms extension that has used the API has been practically discontinued. The XForms accessibility support has also been removed from Firefox 19.


    Map.size and Set.size have been changed to properties

    The size method, that returns the number of key/value pairs saved in a Map object and the number of values saved in a Set object, are changed to be read-only properties.

    Event Handling

    Some event attributes are now limited to body and frameset

    Previously, the onbeforeunload attribute has been recognized even if it has been set on any elements, and the named handler is called when the event is fired. To comply with the spec, it's now ignored when it has been set on elements other than <body> and <frameset>. The other attributes treated the same include onafterprint, onbeforeprint, onhashchange, onoffline, ononline, onpagehide, onpageshow, onpopstate, onresize and onunload.

    File Handling

    The Content-Disposition header's name parameter is no longer supported

    The name parameter included in the HTTP Content-Disposition header used for file downloading is now ignored. This parameter is non-standard and supported only by Firefox and Google Chrome. From now, use the standard filename parameter instead.


    Carbon NPAPI Support has been dropped

    The Carbon event model and the Quickdraw drawing model, deprecated since Firefox 4, are no longer available. Webmasters should make sure your content works well if it requires any special plug-in. If it doesn't work on Firefox 19, please contact the plug-in vendor.

    Document Tags and Contributors

    Contributors to this page: kohei.yoshino, teoli
    Last updated by: teoli,