HTML

Give the browser a clear document, from the doctype to the closing tag.

Build a readable outline instead of choosing tags by their default size.

Understand where a link goes, and why one missing slash matters.

Make an image useful even when it cannot be seen.

Choose ordered, unordered and description lists for different relationships.

Give navigation, main content and supporting information a clear role.

Connect every input to a name people can see and use.

Create a disclosure without building your own interaction from scratch.