This commit:
* Adds default minimum font size limits so that min font size, where provided, does not become smaller than `14px`/`0.875rem`/`0.875em`.
* For font sizes of `< 14px` that have no defined minimum sizes, uses the font size to set the floor of the `clamp()` value.
This bugfix prevents converting existing small font sizes to clamp values that will reduce their font size even further in narrow widths. It therefore improves backward compatibility and accessibility.
Original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/44993#44993 Fluid typography: add font size constraints]
Follow-up to [54260], [54360], [54497], [54500].
Props ramonopoly, andrewserong, isabel_brison, Joen, bernhard-reiter.
See #56467.
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