user6414: I used standard webm settings in VLC player 2.2.1 (the latest) to convert a 4-minute mp4 made in Handbrake. In the conversion panel it says "Streaming 04:00" but the end result is "03:49:15" and, on two further attempts, "03:56:28" and "04:04:23". The file size is almost the same as the mp4 (4MB).
The length of the webm video - i.e. more than 4 hours - is confirmed by MediaInfo and by Firefox native player, videojs player, and VLC player. In VLC player, I tried clicking on the progress bar to explore exactly what takes up 4 hours, but on-click it closes the video. In Firefox, the same test returns the video to frame 1.
Update (30 August)
I did a test on this "generic mp4": http://download.wavetlan.com/SVV/Media/HTTP/H264/Talkinghead_Media/H264_test1_Talkinghead_mp4_480x360.mp4. Do not know if it was done with Handbrake. Then converted it to webm, using default settings, with VLC 2.2.1. 
Results: The mp4 is 00:00:14s, the webm is 22:40:00s. But the webm is not really 22 hours long, it plays for 14s just like the mp4. However, in a html5 player, Firefox and Chrome native players and VLC player, the progress bar gives the length as 22:40:00. 

user1871: Yes, it seems to be a problem of generated timecode. See https://trac.videolan.org/vlc/ticket/12713#no1
Switch to ffmpeg (with a GUI like Avanti) to avoid this issue. My answer to an earlier webm question may guide you on settings.

6414: <p>I used standard webm settings in VLC player 2.2.1 (the latest) to convert a 4-minute mp4 made in Handbrake. In the conversion panel it says "Streaming 04:00" but the end result is "03:49:15" and, on two further attempts, "03:56:28" and "04:04:23". The file size is almost the same as the mp4 (4MB).</p>

<p>The length of the webm video - i.e. more than 4 hours - is confirmed by MediaInfo and by Firefox native player, videojs player, and VLC player. In VLC player, I tried clicking on the progress bar to explore exactly what takes up 4 hours, but on-click it closes the video. In Firefox, the same test returns the video to frame 1.</p>

<p><strong>Update (30 August)</strong></p>

<p>I did a test on this "generic mp4": <a href="http://download.wavetlan.com/SVV/Media/HTTP/H264/Talkinghead_Media/H264_test1_Talkinghead_mp4_480x360.mp4" rel="nofollow">http://download.wavetlan.com/SVV/Media/HTTP/H264/Talkinghead_Media/H264_test1_Talkinghead_mp4_480x360.mp4</a>. Do not know if it was done with Handbrake. Then converted it to webm, using default settings, with VLC 2.2.1. </p>

<p>Results: The mp4 is 00:00:14s, the webm is 22:40:00s. But the webm is not <em>really</em> 22 hours long, it plays for 14s just like the mp4. However, in a html5 player, Firefox and Chrome native players and VLC player, the progress bar gives the length as 22:40:00. </p>
 1871: <p>Yes, it seems to be a problem of generated timecode. See <a href="https://trac.videolan.org/vlc/ticket/12713#no1" rel="nofollow noreferrer">https://trac.videolan.org/vlc/ticket/12713#no1</a></p>

<p>Switch to <a href="http://ffmpeg.zeranoe.com/builds/" rel="nofollow noreferrer">ffmpeg</a> (with a GUI like <a href="http://www.avanti.arrozcru.org/" rel="nofollow noreferrer">Avanti</a>) to avoid this issue. My <a href="https://video.stackexchange.com/questions/15974/how-can-i-reduce-the-size-of-a-webm-to-2mb/">answer</a> to an earlier webm question may guide you on settings.</p>
