If you are using Firefox as your browser and cannot install the PageProof extension, these steps will give you a temporary extension to get your html proofing done.
Download the PageProof extension for Firefox by clicking here.
Go to your Firefox Tools menu and choose Add-ons.
From the cog menu, choose "Debug Add-ons"
Click "Load Temporary Add-on..."
Choose the pageproof-extension.zip file from your Downloads folder
Go to your PageProof proofing screen and refresh.
Note: If you quit Firefox, this temporary add-on will be removed and you’ll need to install it again.