Most Standard way to Set Font Sizes in HTML/CSS

I've been working in HTML/CSS for years, but I'd like to clarify something about setting font sizes. What is the best format to set your font?

Typically, i've been setting with a font-size in a percentage, and then using em to change it up or down from there.

Is this the most standard way to do it? I've seen fonts declared in pixels, points, with relative keywords like "larger" or "smaller" I've seen it set as percentages, etc.

So what's the most standard? Is the most standard the best? any research to back it up?

Thanks,

They're all standard. Use what works for you.