In trying to think of the best solution for iframes and embeds I thought the important thing is that they shrink when the page is smaller than the embed's width... but there are also benefits for letting the user set the embed size manually too. So that's what I did. The width is set in the inline style, but if the page is smaller than the inline style width the iframe will become 100% percent (instead of fixed).
Merge pull request #245 from ucsc/issue-239

Sounds good. I'll have Teresa test some sample use cases.