I recommend you have at least a basic understanding of HTML if you work online. This page provides many useful guides and references for you to help improve your knowledge of HTML.
HTML References and Tutorials
References and extensive tutorials and guides that you can refer to.
Beginner, intermediate and advanced HTML tutorials.
A great collection of HTML tutorials.
A fantastic HTML reference that covers beginner and advanced topics.
The official specification for HTML 4.01.
A comprehensive guide to HTML.
Short HTML Tutorials
The following articles are shorter guides that walk you through how to use HTML.
A step by step tutorial which shows you the basics of HTML and shows you how you can create a simple page with a form, a video, and styling.