About This HTML Issue
The codebase attribute told the browser where to get the code an <object> needed, and browsers ignore it today: the resource URL goes in data and its MIME type in type.
In Flash embed code, codebase holds the address of the Flash Player installer for Internet Explorer, a swflash.cab file on Macromedia's download server, followed by a fragment such as #version=10,0,0,0. Internet Explorer compared that version with the ActiveX control installed on the computer and, when the control was missing or older, offered to download and install the .cab file. Other ActiveX controls, like the ones for QuickTime and Windows Media Player, came with their own .cab URLs in the same attribute.
HTML 4.01 gave codebase a second job: a base path for relative URLs in the classid, data and archive attributes. With codebase="/media/" and data="intro.swf", the file was meant to load from /media/intro.swf. Browsers now resolve data against the page's URL like any other link, and none of them download installers from codebase.
Deleting the attribute clears this warning without changing what the page shows, since browsers already skip it. For Flash embed code, that isn't enough: the classid warning stays, the validator still reports the missing data attribute and the obsolete <param> elements, and the box stays empty because no current browser plays .swf files. Replace the whole <object>, <param> and <embed> elements included, with the same content in a format browsers support, such as a <video> for a movie or an <img> for a static banner.
If the page relied on codebase as a base path, browsers already request the file from the wrong address. Write the full path into data, then remove codebase.
Examples
Invalid: Flash embed code with a .cab installer
<object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" codebase="https://example.com/cabs/flash/swflash.cab#version=10,0,0,0" 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 movie as a video
<video src="intro.mp4" controls width="640" height="360"></video>
Invalid: codebase as a base path for data
<object codebase="/files/reports/" data="annual-report.pdf" type="application/pdf" width="600" height="400">
<a href="/files/reports/annual-report.pdf">Download the annual report (PDF)</a>
</object>
Valid: the full path in data
<object data="/files/reports/annual-report.pdf" type="application/pdf" width="600" height="400">
<a href="/files/reports/annual-report.pdf">Download the annual report (PDF)</a>
</object>
Find issues like this automatically
Rocket Validator scans thousands of pages in seconds, detecting HTML issues across your entire site.