Apps similar to mailmouse include Total Validator, which has a free version. Other options are HTML Validator. Between them they cover Mac, Windows and Linux.
Total Validator is an HTML validator, an accessibility validator, a spell checker, and a broken links checker all rolled into one tool, allowing one-click validation of your website.
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing. The details of the errors are seen when looking the HTML source of the page.
