Mobile phones Font Sizes are based on a Default size "medium" defined by the Manufacturer. Some phones also have a smaller font and some have an additional larger font. There is no standard so you can not define your "text-size" in Pixels, % or ems.
Headings h1 -h6 are based on three font sizes. h1 = large + bold. h2 = large. h3 = medium + bold. h4 = medium (Default). h5 = small + bold. h6 = small. Margins for h1/2=10px h3/4=8px h5/6=6px (Nokia 60). Not all work on all phones.
Bold is not always available on all fonts especially "small"
Font-size:medium; is the Default size. Other sizes fromm xx-large to xx-small work in the same way as h1 - 6 but without the margins. Pixel heights vary between phones from 0.14 to 0.27mm and they may not even be square. So you have little chance of producing a consistant display.

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