I'm converting Flash movies to Mp4 (and webm) and having to learn lots about web video.
The process starts with MOVs exported from Flash. For example, I have a MOV of 640px x 290px, 1m 58s, 2843 frames, 24fps, 171 MB, 11 Mbps.
Converting that to H.264 with Handbrake, with constant quality of 20 RF, I get a file of 3.45MB with - according to MediaInfo - "overall bitrate mode: variable", and an "overall bitrate" of 240 kbps (video 79, audio 144 with max 170). Bitrate Viewer tells me the average bit rate (this might be video only) is 92 kbps, with a 1-second peak of 1019.
Then I tried Handbrake's variable bit rate, because (if I've understood its advantages correctly) there are 20 seconds with lots of motion, in the middle of the video, which might look better with a higher bit rate. With VBR 300, I get a 6.3 MB file with (MediaInfo) an overall bit rate of 440 kbps, video 300, audio 144. Bitrate Viewer has average 293 kbps, with that peak now at 3467. For about 65% of the video it is below 130 kbps. I've placed the Bitrate Viewer profiles here: http://www.casedasole.it/bitrates.html
With Flash, bytes per frame were important because, when streaming, you needed to have the browser download as much as possible in advance, so that particularly big chunks of bytes (like a big JPEG) were already loaded before the Flash player reached them.
My question is: Is that the same principle with video bit rates? Looking at that peak bitrate of 3467 (converts to 433 kB), I think: "That's OK, by then, the whole video will be loaded". Or am I missing something?