H1 is the HTML element commonly used to identify the most important header in a web page. Examples of H1 would be the title of a page, the title of a post, the name of a product, etc
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The HTML language allows us to establish different headers according to their importance. Specifically, in a web page we can create from H1 to H6, the first being the most important and H6 the least important.
Although we are used to having only one H1 per page, there are common cases in which there may be many of them. Let's think, for example, of the cover of a blog or a product category page in an online store. In these two cases, each of the post titles and product names would be within an H1 tag.
We can talk about their importance on two levels: importance to users and importance to search engines.
Summarizing, headline tags (H1 being the most important one) are very important to have an adequate information structure and a good user experience, since they define the content hierarchy, something that will improve SEO considerably. In addition, the H1 is usually at the top of the content and, if it includes the keyword, more importance will be given to it by Google.