I can eliminate width and height in the source code and that works - until I save the article the next time. Then the dimensions
are inserted back again.

If you remove the dimensions in the Code tab save, the dimensions won't be added back. Thay will only be added again if you edit the video with the Media Manager dialog again (which I will fix in the next update).

Assuming you are using JCE Pro 2.9.38?

Any reason why you don't want to include the dimensions?

The dimensions are also re-inserted if I switch back from code view to standard view of the editor and save the article. So for me the problem does not only occur if I open the Media Manager... Any ideas?

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If dimensions are inlcuded, the video doesn't fit correctly. Especially if the site gets smaller the occupied space by the video looks odd. If no dimensions are inlcuded everything looks as it should.

Try adding the following to your template stylesheet:

video {
    max-width: 100%;
    height: auto;
}

This will preserve the aspect ratio of the image while scaling it down to fit the parent container div.

Just to clarify, there are reasons why you would want to include dimensions for images and video, most notably:

Layout stability: Specifying the dimensions helps other elements on the page to position and flow correctly around the video. It prevents unexpected layout shifts that can occur when the video loads or when the page dynamically adjusts its content.

Accessibility: Providing explicit dimensions allows assistive technologies to understand and convey the video's size to users with disabilities. This helps in creating an inclusive experience for all users.