Tools for Validating Mark-Up & Doing Cross-Browser Testing

Found two great tools this afternoon for spotting issues in mark-up & testing websites across multiple browsers & devices:


Did anyone ever find any free cross-browser testing sites ( only free on CodePen pens) that, like Browser Stack and CBT, actually emulate devices?
(Free trials on these sites are really limited, BTW.)

If you’re running Windows, you can pretty easily install most of the browsers you need locally on the computer (Windows 10 comes with Edge and IE11, so you only need to install Firefox, Chrome, and Opera), and Microsoft offers VirtualBox images that have Windows 7 with old versions of Internet Explorer if you need them—which you can also use on a Mac or Linux, if you need to test on IE. The only major wrinkle is if you’re running Windows or Linux and need to test on a Mac with Safari—and I don’t know of any good free or cheap solutions for that (the best would probably be to get something like an iPad).