HTML validators are invaluable tools for people that write code. They can let you know if there are any errors present that might cause display issues and give you some clues for debugging. Validators do this by checking your code against a specification, or a set of rules for the language that your code was written in. These specifications make up what is commonly referred to as “web standards.” The trouble is that most email clients don’t support web standards and instead only support a hodgepodge of HTML and CSS. Validating HTML for email can be tricky–read on for our how-to guide.