If you're the GM, you could make a .gif of the animated text, upload it, then place it as a token somewhere the players can't see it. When you want to display it, select the token and hit Shift-Z to display it for all players - it will pop up in original size/ratio in front of the canvas. If you're not GM, you're limited to displaying things in the chat log - but you can send .gifs to the chat log or put them in templates, and send them to chat via macro. If you want actual flashing text.... I think that went out of fashion in the Netscape days?
