We knowl examples, proofs, and inline exercises by default, mostly so a new author knows that presentation is an option. The knowlization of items can be controlled by a variety of command-line processing parameters (Section 3.1). While these are in flux, we do not list them here, but instead suggest you open xsl/mathbook-html.xsl and search on html.knowl to see what is available.
Note that some of these switches are for broad categories of items, for example, html.knowl.theorem will also knowl <lemma>s, <corollary>s, <fact>s, and more. This choice applies document-wide, there is no plan to support electing this on a case-by-case basis.