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HTML might be getting a new type of tag, which… hasn't happened this millennium. Here's the new syntax, and how it works:

You said your example causes a layout shift, but if you're streaming in HTML and the browser is laying out the first content would it be possible *not* to cause a layout shift?

you could be putting the new content in a non visible area (eg: that needs to be scrolled to or expanded), or if you can anticipate them you could set fixed heights to parent elements to reserve space on the screen for the new content

or the content below the <summary> in <details>