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The picture and srcset attributes allow responsive image selection.
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The details andsummary elements enable authors to provide extended information that users can choose whether to read.
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The menuitem and type="context" attribute value enable authors to add functionality to the browser’s context menu.
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The requestAnimationFrame API allows for more efficient animation.
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enqueueJob and nextJob help explain Promise resolution in terms of microtasks.
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The rev attribute for links, primarily to support RDFa (previously defined in HTML 4).
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HTMLMediaElement and srcObject objects.
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Enable cross-origin track and EventSource and cross-origin content for ImageBitmap in canvas.
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event-source-error, event-track-error and event-track-load events for media fetching.
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onrejectionhandled and onunhandledrejection and APIs for tracking promise rejection.
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HTMLTableCaptionElement, HTMLTableSectionElement, HTMLTableRowElement, for HTML table elements.
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history.scrollRestoration to control where a users' view is directed when navigating through their history.
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IDL [SameObject], for some objects that return collections.
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Add "noopener" to rel and window to allow for browsing contexts to be separated.
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nonce attribute on script and style to support the use of Content Security Policy.
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appCache.
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Media Controllers.
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The command API.
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The usemap attribute on object.
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The accessKeyLabel IDL attribute.
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The form attribute is no longer valid for label.
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The multiple attribute on input type="range".
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hreflang andtype attributes on area.
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The use of nested section elements each with an h1 to create an outline.
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Special handling of isindex in form submission.
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navigator.yieldForStorageUpdates() and the Storage mutex.
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Disallow tfoot before tbody.
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[Exposed=Window] for HTMLHyperLinkElementUtils, [Exposed=Window, Worker] for DOMStringMap and IDL Date.
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The accesskey takes a single character as a value (as in HTML 4).
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header and footer elements can be nested, if each level is within a sectioning element.
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option elements can be empty.
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The mousewheel event is called wheel.
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The value attribute of input type="submit" is translatable.
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A figcaption can appear anywhere within a figure.
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Having title, or writing email to a friend does not make an img missing alt conformant.
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The content of time is phrasing content, or text.
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Blank alt on area elements with duplicate href attributes is no longer conformant.
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When navigating internally, the next search for a link etc starts from where the navigation moved to.
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img and related elements support width="0".
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.tFoot and .createTFoot() always insert at the end of a table
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fieldset and namedItem make HTMLCollections not HTMLFormControlsCollections and HTMLOptionsCollections.
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frameElement can return null.
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For images which don’t resolve currentSrc is the URL given to the resolver, not necessarily the absolute URL.
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script IDL attributes reflect.
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meta refresh allows ; or url= to be optional.
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navigator.javaEnabled() is a method.
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fileCallback is called blobcallback.
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The toBlob() callback is non-nullable.
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origin on HTMLHyperlinkElementUtils and Location is readonly.
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The first title child of an SVG is its title, not the last.
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window.open() can return null
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