Websites have different units of measurement to express ‘length’, all of them (px, %, EM, etc.) are supported by RapidUI.
There are many properties that use ‘length’ values, like width, height, padding, margin, font-size, border-width, etc.
Choosing the right unit of measurement will help you create better sites and achieve responsive layouts easier.
It is recommended to use px (pixels) and % (percentage), depends on your designs and desires.
In most sites, you can easily achieve pixel-perfect responsive results with px units only, (without %), by setting the center of your site (read full guide), and using 'Scale Content' to create a breakpoint (read full guide).
Some sites require both px and % on the same page to create full-width responsive layouts, or by setting many breakpoints (which is really not recommended in most cases).
You can also use different units in separate breakpoints, to create custom responsive layouts.