The `font` tag is obsolete in HTML5, so we are taking the risk that browsers won't implement it anymore. Instead of controlling the font-size this way, I recommend adding different classes for each label with the `cls` attribute. Afterwards, you can set the font-size in CSS. 

For `font-size` units, please use REM as it's considered a best practice. You can read a bit about REM here: http://css-tricks.com/theres-more-to-the-css-rem-unit-than-font-sizing/.

For classes names, I suggest something like this:

```css
.font-size--big {}
.font-size--small {}
.font-size--medium {}
```

In that case, each `cls` attribute should contain the corresponding class.

Other people use a different approach for CSS classes, such as `alpha`, `beta`, `gamma`, `delta`, etc. You can find an example here: https://github.com/csswizardry/inuit.css/blob/master/base/_headings.scss.

Pick the one that you like the most :)