Quoted:
Large animated gifs take forever to load, slow down or crash certain browsers (especially mobile browsers), and still end up looking like blurry washed-out crap. WebM files are far smaller despite having higher resolutions and framerates, and also have sound support.

Basically animated gif files are now obsolete.

The thing is, is that you need a little bit of extra code in order to embed a webm file; it can't just be posted like an image as the browser needs to know what codec to use to try to display it. This ought to do the trick:

<video width="640" controls>
 <source src="URL_GOES_HERE" type="video/webm">
</video>

If there was a button on the AR15.com editor that would take a URL, verify that it has a .webm extension, and plug it into the above code that's all we'd need. Seems to me that it wouldn't be too hard to copy and modify the existing code for [url=http:// tags to do this. Just have a [webm]http://] tag basically.

Just to give an example of what WebM is capable of as compared to animated gif, this video is just under 4mb.

http://a.pomf.se/kfsske.webm

this is an animated gif of approximately the same file size:

http://a.pomf.se/duqeai.gif

For a more AR15.com relevant example:
http://a.pomf.se/sbhcdr.webm vs.
http://a.pomf.se/muxmiw.gif (both around 2mb)

Seems to me that giving AR15.com WebM support would require very little effort in order to allow the use of a vastly superior format which is quickly replacing gif as the default short video format on the internet.