There are 6 tags that define a heading in HTML.  These tags are designated from H1 to H6.  By default, the H1 tag is the largest or most important heading while the H6 is the least.  All of these tags are supported by all major browsers.

As with all search engine optimization efforts, no matter how small or seem insignificant, proper programming and page layout, aside from providing informative and effective content should always be practiced.  Headings are generally used to define the content of the page and for proper page layout.  Every written content or article should have proper headings.