I downloaded a YouTube video on a Ubuntu Linux with yt-dlp. When I tried to view the downloaded WebM file with the default video application, Videos, also known as Totem, which is an application for playing videos that is provided with Ubuntu, I saw the message below:

When I opened the video with VLC, which I had previously installed, I heard audio, but no video was displayed. I checked to see if the Nvidia driver was up-to-date since the system had an Nvidia graphics card, but they were up-to-date. You can check that by clicking on the Show Apps button at the lower-right-hand corner of the screen Ubuntu desktop. Then type Software Updates in the "Type to search" field. Click on Software & Updates and then click on the Additional Drivers.

To resolve the problem, I added the following line to the /etc/environment file with a text editor:

GDK_GL=gles

When I rebooted the system, I was then able to view the video with the default Videos application, though when I tried playing it in VLC I could hear audio, but did not see video.

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When I tried to play a YouTube video I downloaded as a WebM file using yt-dlp on an Ubuntu Linux system in the VLC media player application, I could hear the audio, but no video was shown. I tried opening a few other .webm videos and found I was able to view the video in some, but not others, though I was able to view the video and hear the audio in all the .mp4 and .mkv files. I tried changing the video output setting from "Automatic" to "OpenGL video output" and then "XVideo output (XCB)", which you can do by clicking on Tools, then Preferences, then Video, and then selecting those options from the dropdown list next to Output, but neither of those options resolved the problem, so I reset Output to Automatic. I was able to resolve the problem by taking the following steps, though:

1. In the VLC program, click on Tools and then select Preferences.
2. Click on Input/Codecs and change the hardware-accelerated decoding setting by clicking on the downward arrowhead next to Automatic, which will be the setting if the default setting is in effect.
3. Change the setting to Disable and then click on Save.

When I checked the Codec information for the file where I could not view the video before disabling the hardware-accelerated decoding setting, which you can do by opening an audiovisual (AV) file in VLC and then selecting Tools and Codec Information, I saw the video codec was AOMedia's AV1 Video (av01). When I checked the video codec for another WebM file where the video wasn't visible in VLC, I saw it was also the AOMedia's AV1 Video (av01) codec. Checking the video codec for .webm files where I could view the video, I saw Google/ON2's VP9 Video (VP90) for some and H264 - MPEG-4 AVC (part 10) (avc1) for another.

Note: you can also determine the codecs used in a file using the mediainfo utility, which you can install on an Ubuntu system with sudo apt-get install mediainfo. You can check on whether it is already installed using the command which mediainfo, which would show the location of the mediainfo executable file if the application was installed, or you can use dpkg -s mediainfo, which will show details on the mediainfo package if it is installed.

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