Thanks for your answer, @otto42.
Well, switching back and forth between block visual/HTML mode wasn’t the best experience for me.
I just wanted to add an ID attribute to a Cover Image element (as it can be used as a separator and I wanted to add some in-page navigation based on IDs), but after doing it in HTML mode I wasn’t able to switch back to the visual mode.
Using simply the HTML block is not an option, as my client – who doesn’t know HTML – will want to modify the background element and the text on the cover image occasionally.
And there was another thing with the cover image element: that despite it’s visually big, it uses P element for the text. For the sake of SEO it would be much better if we could choose H1..H6 headings here.