Software Design by Example in Python 11: An HTML Validator

This book teaches software design by building scale models of tools that programmers actually use. It therefore seemed fitting to spend Chapter 11: An HTML Validator on a tool that I built for the book itself: one that compares the structure of its HTML pages to a specification of what elements are allowed and what attributes they can have. I have written several such tools over the years to check the output of various static site generators. Each time, I wished that I had started earlier and been more thorough, both so that the generated pages actually matched my CSS and so that HTML-to-LaTeX conversion would work correctly.

This chapter was also an opportunity to introduce the Visitor pattern. It comes up several times later on, but so much else is going on in each of those examples that explaining visitors would push the content over the one-hour limit. Once again I was struck by how much instructional design has in common with writing fiction: if you want something ready for Act 3, you need to figure out how to introduce it in Act 1 or Act 2 without being too obvious.

Terms defined: attribute, child (in a tree), closing tag, DOM, DOM tree, element (in HTML), HTML, node, opening tag, self-closing tag, tag (in HTML), tree, Visitor pattern.