Headings are subtitiles of sections in a Web page. HTML defines six levels of heading elements, H1 to H6. When displayed by a browser, the plain text in heading elements becomes bold, with different sizes. However, you should not use heading elements to make text bold or change font size. As the name implies, use them for headings of document sections only. Different heading elements let you specify the hierarchical structure of your document. The H1 element has the highest level, followed by H2. H6 is the lowest level.