How to handle HTML/CSS font sizing for desktop browsers and mobile browsers?

I have a couple questions relating to setting the font size using em, pt, px and/or percent.

If your web application is for a desktop browser only then what do you use: em, pt, px and/or percent? If mobile only? If you need to support desktop and mob开发者_JAVA技巧ile?

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You should almost always use fluid font sizes.

Users need to access your content. If they cannot adjust the font size to read the content, then the application is not useful for them.

It should not matter whether this is for a desktop browser or a mobile device.