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.
To add the PageProof extension to Firefox
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.