There is a litte bash script called lightsOn on GitHub, which automatically checks on various apps, including fullscreen Flash and html5 video. (It works for me.)
Bash script managing screensaver and display power management (DPMS) on different conditions (fullscreen videos, specific applications, specific outputs).
To use the program save the file
lightsOn.sh in your home directory, make it executable by typing
chmod +x ~/lightsOn.sh and run it
~/lightsOn.sh 300 (this will check every 300 seconds = 5 minutes).
If it works you can add the script to startup by adding
~/lightsOn.sh 300 in System Settings > Applications > Startup.
The script detects a variety of apps / usages, you can configure (among other options) detecting certain things right in the shell script:
# Modify these variables if you want this script to detect if MPV, Mplayer,
# VLC, Minitube, Totem or a web browser Flash/HTML5 Video.