Is this possible? If those HTML buttons were attached to hyperlinks? Possibly through generating WebAccess links?
It is possible to embed HTML buttons in the email. However, you will have to build the web service to handle the responses from those buttons, neither Workflow, nor Web Access have any functionality that allows approving through a URL.
If you have LF Forms, this is actually somewhat possible. What you do is configure a form that you can fill with whatever pertinent information.
Then you have workflow prepare your email and prepare a URL with values being sent through to the form as the URL for the Approve or Deny buttons.
When the user clicks either of the two links, it will open up your form, and change a field that will indicate approval or denial. You will need to use Javascript to take the value that gets filled into that field and instantly trigger the appropriate submission button for the whole process to work. Or you just have the Javascript click "Submit" and have that field you filled be the deciding factor.
You can also have the link fill a field that will allow Javascript to display an iFrame with the document in question so they can review it there if you do not want to automate the process.