In reply to FesteringSore:
1) Do you really want to? They can be annoying and have a certain 90s look to them
2) A fluttering flag is a video and there are many ways of showing simple videos.
a) The best bet is probably an animated GIF. e.g.
http://www.crossed-flag-pins.com/animated-flag-gif/flags-Great-Britain.html . Probably a pain to make, but lots of premade ones about on the internet if you want a common flag.
b) You could also use javascript to make a flag then make it move:
http://phrogz.net/tmp/canvas_flag_wave2.html
c) Flash would work, but flash is a terrible thing and should die, IMHO.