SW64, thanks for both of those great tips! This is the ffmpeg for Slackware 14.2, so it was apparently not built with libx264. I'll certainly give ffmpeg a try as a recorder.

Meanwile, I believe I've found a solution that will work on Windows: OBS Studio, which is a free download. It allows me to do all the fps and resolution type settings discussed in this thread and, most importantly, the browsers can play it. By experimentation I've found that I can set the resolution to 820x514 for the minimally decent viewing resolution (native is 1920x1200). OBS is set to have a "canvas" of 1097x686, although I'm not sure what that means yet. These setting appear to create files at 9M per minute, which would be a usable file of 63M for my 7 minute video, with no need for ffmpeg post-processing. I'll re-record my 7 minute video tomorrow and post back size results.