"Dynamic HTML" is a bit of a marketing phrase; do they get more specific than that?

By the way, there is a United app for both iOS and Android which lets you retrieve your boarding pass and store it in the app so it is available even if you lose your wireless connection. I've only tried it once -- today, on Android -- and everything went very smoothly. (By contrast, the email message required me to have a live browser connection to launch the boarding pass, which I didn't want to risk outside the wi-fi coverage.)

http://itunes.apple.com/us/app/united-airlines/id449945214

https://play.google.com/store/apps/details?id=com.united.mobile.android&hl=en

Try clearing your cache/cookies and try again. I experienced a similar with trying to print a boarding pass on United and clearing the cache resolved it for me.