Final answer:
The very first tag when creating a document in HTML is the declaration, followed by the tag which encompasses all page content.
Step-by-step explanation:
The very first tag that is used when creating a document in HTML is the declaration. This declaration is not actually an HTML tag; rather, it is an instruction to the web browser about what version of HTML the page is written in. Immediately following the declaration, the tag is used to begin the HTML document. This tag encompasses all the content of the page, including the and sections where further tags and content will be nested.