TiddlyWiki actually handles video/webm as a tiddler content type! So, instead of using the <video> HTML tag, try this:
1) Import a webm file into your TW. This will create a tiddler containing binary data encoded into base-64, with type="video/webm"
2) If you view that tiddler, you will automatically get a <video> element that is sized to fit the tiddler.
3) Use the <video> control to play the video.
4) To embed the video in another tiddler and scale it to fit a specific size, you can use transclusion, like this:
<div style="width:320px;height:240px;">
{{NameOfVideoTiddler.webm}}
</div>
Note: if you omit the height:... attribute, the video will automatically scale in proportion to the width.
enjoy,
-e