The very first header of a document can be of any level, although we highly recommend your document to start with a
\H[1], and to contain exactly just one \H[1], as this has implications such as:\H[1]is used for the document title: HTML document title\H[1]does not show on the table of contents
After the initial header however, you must not skip a header level, e.g. the following would give an error because it skips level 3:
= my 1
== my 1
==== my 4