No offense, but everything you say relating to heading tags is gibberish nonsense.
It's not H1...H5, the lowest order heading is H6. They're not about "making a page pop" and if you are thinking about them in terms of what they look like, in terms of 'largest to smallest" then you are utterly and completely devoid of the most basic of HTML knowledge.
The (singular) H1 tag (signular) is used as the heading (singular) that everything on every page of a site (or app) is a subsection of. Thus usually your H1 should be the site title. Reemember, <title> is for the UA, not the user and is not considered content.
An H2 is the start of a major subsection of the current page, with the first H2 marking the start of your main content if you don't use the MAIN tag. H3 marks the start of a subsection of the H2 preceding it. H4 marks the start of a subsection of the H3 preceding it. I would hope not to have to explain what H5 and H6 means. Just as HR does not mean 'draw a line across the screen', it means a paragraph/heading level change in topic.
IF you are choosing any of your markup based on appearance, you're no designer.
As evidenced by your having fairy tale made up tags like EMS or REMS. There's an EM tag which means "emphasis" and not "show this in tailic", but there's no such thing as REMS or EMS. Are you confusing CSS properties with HTML tags? (em/rem respectively)
That you're further screwing things over talking about PX measurements, and a general attitude of "screen media screw everyone else" makes me think that you might have fallen into the trap of being an artist who has deluded oneself into thinking you know what design is.
As evidenced by gibberish like "zoom-scale attributes" and then having tags? Delta Alpha Foxtrot Uniform Quebec?!?
Actual design is about more than what things look like. It involves specifications, guidelines, usability, accessibility, and even non-visual behaviors.
Thus if you are designing for the web using web technologies and start out dicking around in glorified paint programs like Photoshop or Figma, you're no designer.
Actual design starts with content or a reasonable facsimile of futre content in a flat text editor as if HTML, CSS, appearance, or the rest of it doesn't even exist. You arrange it in a logical order, then you mark it up sematnically to say what things are grammatically / structurally. (so no DIV or SPAN at this point since they mean nothing). Then you move on to adding your screen media stylesheet to bend that markup to your will, adding DIV/SPAN and classes only where/as needed, creating your responosive layout.
Then and only then does one do the artsy-fartsy colours and paintovers.
STarting out screwing around with screen appearance is not design, no matter how many delusional artists claim otherwise. That stuff should be at the end of the screen (and/or) print sheet creation process, not something you waste time dicking around with at the start, flipping the bird at everyone who isn't perfectly sighted sitting at a high res graphical display.
Design is not art unto and of itself.