Hey, Jeff. I'm following so many iWeb threads right now I'm not sure which was the start of this one. I have three observations:
First, there seems to be inconsistency about whether iWeb works properly with El Capitan, or not. I have lots of websites and find that even among them there are problems on some and not on others. On several sites images [either my own photos or from the Insert Shape function in the menu bar] once they are linked become invisible when published, even though the link is still active when you mouse over it. When the page loads there is a brief flash of the outline, but the image does not appear. Remove the link, republish, and it's visible again. As I suggested above, the scariest part is, it does not occur on every website. But I can't take chances, so I've moved all my websites to an iMac still running Yosemite.
As for the videos issue, the topic is more complex than can be blamed on iWeb. Different browsers nowadays require different formats, so when I was embedding code in the html snippet I had to insert code for both .mp4 and .webm formats. [I learned this from the ever-useful Cedric Giger with his iWebFAQ.org although I decided his approach is now out of date, too. Still, it's worth checking out his site to understand the principle of videos being viewed on different browsers.] In the end the really good solution is using Vimeo [or YouTube, although I clearly prefer Vimeo]. Not only is all the code correct for all browsers, you're not clogging up your own server with huge video files. In my work I need both HD and Closed Captioning, so again that comes easy with Vimeo.
As for EverWeb, everybody seems to be talking about it. It has great features that iWeb should have had from the beginning, yes, but it also lacks important features that iWeb has. It's a program that's still learning to walk. We're all hoping that the EverWeb folks will listen to our observations [or perhaps hire Cedric as a consultant] but at present it is not as sophisticated as iWeb.
Unlike my other, more skeptical colleagues out there, I'm still counting on Apple to save the day, show their human side [ha, ha...] and come up with a new iWeb.
Richard