Muy importante, aprende a usarla correctamente:
The HTML <main> element represents the main content of the <body> of a document or application. The main content area consists of content that is directly related to, or expands upon the central topic of a document or the central functionality of an application. This content should be unique to the document, excluding any content that is repeated across a set of documents such as sidebars, navigation links, copyright information, site logos, and search forms (unless, of course, the document’s main function is as a search form).
Note: There must not be more than one <main> element in a document, and it must not be a descendent of an <article>, <aside>, <footer>, <header>, or <nav> element.