Priority works depending on the closest enclosing tag for the text.
<font color="rgb(255,255,255)">
Testing
<font color="rgb(255,0,0)" size="10"><font size="16">foo</font></font>
bar
</font>
This would create Testing foo bar where foo is red with size 16, not 10
I don’t think a priority attribute is necessary for tags, if you want to not apply a specific item to a piece of text, you should not enclose it with tags