The H3 tag is an HTML element that defines the third-level heading (or subheading) on a web page to mark up a subsection of an H2 heading in the content. A web page can have any number of <h3> tags to break up the subsections of the <h2> subheading topics.
H3 tags should contain information that defines the subtopics of the H2 subheadings that require additional details to help the reader understand the content. An H3 tag carries less value for on-page search engine optimization (SEO) than the H2 header tag but more weight than the H4 tag. A good practice for SEO is to include variations of the target keywords, synonyms, and semantically-related words in the H3 header tags to improve topical relevance for the main keywords while also avoiding over-optimization for the primary and secondary SEO terms.
Putting relevant keyword variations, synonyms, and semantically-related words in the H3 header tags reinforces the topical depth of the content, which search engine crawlers analyze for proper indexing and ranking in the search engine results pages (SERPs).
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