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Thanks Bibblehub, great idea! I'll test it out next time I try Google Chrome.
Carlton - Bibblehub means type '20 th', right-click 'th', choose 'Add to dictionary', then delete the space to make '20th'. '20th' will no longer be marked as a spelling mistake, and neither will 4th, 5th, 6th etc.
It seems rather careless of them to release it with a spell checker like that. Even more surprising is that 'Google' is marked as a spelling mistake.