I am building a video-site for long lecture recordings. So a streaming server is a must. We only have to support newest browsers, which support the HTML5
<video> tag. So when the stream is done via RTMP protocol (Red5 server for example), will clients have to have adobe flash installed or is a browser capable of the
<video> tag enough?
Best How To :
I think that for a simple video streaming service (for VOD) you are not forced to use a streaming server (like AMS, Wowza, Red5, ...) just a HTTP server (like Apache, nginx, ...) with a good configuration and some special modules (like nginx ngx_http_mp4_module, H264 Streaming Module for Apache, ...) can do the job because it can provide a HTTP stream available for Flash Player and HTML5.
So for your question, you should know that to play an RTMP stream on a browser, you should have Flash Player, HTML5 can not play it. That's why you have to provide a HTTP stream for HTML5 based video player. After that, you can use a Flash to HTML5 fallback or the inverse.
Hope that can help.