It looks like you are running into Issue 62.
Solution 1 - Upgrade Watir-Classic
This bug is fixed in the latest version of watir-classic. Note that you need to upgrade the watir-classic gem (not just the watir gem):
gem install watir-classic
Solution 2 - Use an Array
If upgrading to the latest version is not an option, you can make the :tag_name value an array:
browser.td(:id, 'Proceed').element(:tag_name => ['FONT']).click
Solution 3 - Use font method
I just double-checked and the font
element is actually supported. So I would say the better solution, for this specific case at least, would be to use the font
method:
browser.td(:id, 'Proceed').font.click