I am reading and taking notes on the HTML specifications for 100 days as part of #The100DayProject. Read the initial intent/backstory. I am a Microsoft employee but all opinions, comments, etc on this site are my own. I do not speak on behalf of my employer, and thus no comments should be taken as representative of Microsoft's official opinion of the spec. Subsections not listed below were read without comment.
Currently reading in 3.2.4 Content Models.
126.96.36.199.6 Embedded content
"Embedded content is content that imports another resource into the document, or content from another vocabulary that is inserted into the document."
Includes audio, canvas, embed, iframe, img, math, object, svg, and video. On occasion (spoiler alert!) the spec will define fallback content for these elements.
188.8.131.52.7 Interactive content
These elements have an activation behavior, "normally culminating in a click event". The user can trigger a synthetic "click" by other means, such as voice input or keyboard navigation.
If a click() method triggered the synthetic click, "the isTrusted attribute must be initialized to false". Seems like a security measure.
If you ever wondered how fussy this stuff is, here's a good example: "Click-focusing behavior (e.g. the focusing of a text field when user clicks in one) typically happens before the click, when the mouse button is first depressed, and is therefore not discussed here."