To be an effective HTML developer, you need to be proficient in HTML and HTML5, understanding all the tags, attributes, and elements of the language. This includes knowing how to format text, create links, tables, forms, and embed multimedia. Understanding of CSS is also vital, as it works closely with HTML to style and lay out web content. Basic knowledge of JavaScript can be helpful to make static HTML more interactive. Additional skills include an understanding of SEO principles, familiarity with web accessibility standards, a keen eye for detail, and problem-solving abilities. Experience with development tools, version control systems, and debugging tools is also beneficial.