<blink>This is the world's most-hated HTML tag.</blink>
I don't think Internet Explorer or Chrome supports this tag anymore. It works in FireFox.
Here's a way to achieve this, though: add an additional Property Set (make it a copy of the Property Set you want to flash), set the Conditions to time % 2 = 0 (using Advanced conditions), set the Font color to "transparent", then move the new Property Set just above the existing set.
As an example, I did this with the "Round" cell on the default layout. This cell has a Default Property Set and an "Even Rounds" Property Set. I made a copy of the Default Property Set and performed the above steps and it flashes on for one second, off for one second, on for one second, etc.
Of course, during an even round the "Even Rounds" Property Set will be selected, so it won't flash. To make it flash I would just need to duplicate the above steps for the Even Rounds Property Set as well.