The “classid” attribute on the “object” element is obsolete. Use the “data” and “type” attributes to invoke plugins.

The classid attribute told the browser which plugin or ActiveX control should run an <object>, and HTML has dropped it: the resource URL goes in data and its MIME type in type.

Its value was usually clsid: followed by the ID of an ActiveX control, and only Internet Explorer could load those. A typical one is clsid:D27CDB6E-AE6D-11cf-96B8-444553540000, the Flash Player control. Flash embed code usually pairs it with a codebase attribute pointing to the Flash installer, a list of <param> elements naming the .swf file and its settings, and a nested <embed> for other browsers.

Browsers no longer look at classid. An <object> loads the URL in its data attribute, and when data is missing it renders the content between its tags instead. For a Flash embed, that content is the <param> elements, which display nothing, and the <embed>, whose .swf won't play. The validator also reports the missing data as an error next to this warning.

The message still mentions plugins, but current browsers don't run any, and Adobe ended support for Flash Player on December 31, 2020. Moving the .swf URL into data with type="application/x-shockwave-flash" clears the warning, but the box on the page stays empty. Replace the content itself: a video file goes in <video>, a video on a hosting service goes in that service's <iframe> player, and a banner can become an <img> or a short video. Then delete the old <object>, its <param> and <embed> elements included.

When the <object> holds a file that browsers handle without a plugin, such as a PDF or an SVG image, keep the element and remove classid. Point data at the file and set type to its MIME type (application/pdf or image/svg+xml). Anything inside the element, like a download link, shows up when the browser can't display the file.

Examples

Invalid: a Flash embed

<object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" width="640" height="360">
  <param name="movie" value="intro.swf">
  <embed src="intro.swf" type="application/x-shockwave-flash" width="640" height="360">
</object>

Valid: the Flash movie converted to a video

<video src="intro.mp4" controls width="640" height="360"></video>

Invalid: classid left on a PDF embed

<object classid="clsid:CA8A9780-280D-11CF-A24D-444553540000" data="report.pdf" width="600" height="400">
  <a href="report.pdf">Download the report (PDF)</a>
</object>

Valid: data and type only

<object data="report.pdf" type="application/pdf" width="600" height="400">
  <a href="report.pdf">Download the report (PDF)</a>
</object>

Learn more:

MDN Reference

MDN - object element

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MDN Reference

MDN - video element

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WHATWG Spec

WHATWG - Obsolete features

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