Font Size Conversion Chart
|Points assume a standard 96dpi viewport,
with a :root font-size equivalent to 16px
If your document is set at 72dpi (or 72ppi) then while inside Photoshop points = pixels but that's not true in a browser. For clarity always state sizes in pixels to web developers who should convert to em or rem.
For example on my 96dpi laptop, using Firefox Developer v64, 1 inch (25.4mm) of viewport width (measured with a rule) = 128px (pixels) = 96pt (points) = 8pc (picas). Technically that shouldn't be the case, but it is.
These days (2018) It's best practice to let the device / browser make the base font decisions. As far as I've seen, on a 96dpi viewport, they default to the equivalent of 16px but I doubt that applies to an Apple watch.
When setting a font size I mostly use the rem unit, which references the pages root
font-size and eliminates cascade issues. Though in the rare case where I'm relying on the cascade, I fallback to em.
Personally I'm very careful with line heights and lengths / widths.
Ideal line width for scanning or reading still remains the same at 45—75 characters for paragraphs which works out at 34rem approx.
Line heights should increase as the width decreases, which does sound counter intuitive. With a base
line-height: 1.5 it will only change by ±0.065 between 320px (20rem @ 16px) and 34em (544px @ 16px) so, unless you're a perfectionist, it could be ignored.
The minimum font size should never be smaller than 10px. And that's for small print, asides, captions, or the like and not body copy itself. I'm very pleased to finally see font size increasing generically across the web.
height is not set on text blocks. By all means use
min-height but if the user increases the font-size by 200%, for accessibility, a set height is likely to crop the text and the page fails WCAG.
A third of your visitors need glasses!
In fact some two thirds of your audience need help viewing websites. Amazingly half of them don't wear their glasses or contacts when browsing online.