rlh
The root element's line — the same lh, immune to context.
One rlh is the line-height of the root element, the html — the same measure as lh, only anchored to the document's staff instead of the local one. It stands to lh as rem stands to em.
It is the unit of global vertical rhythm: margins and gaps in 1rlh align headings, paragraphs and captions of different sizes to the same grid of lines, like a notebook's ruling running across the whole page.
* + * {
margin-block-start: 1rlh;
}