Final answer:
REMs size is relative to the font-size of the HTML element. REM stands for 'root em' and is a scalable unit used in CSS for consistent sizing across a website.
Step-by-step explanation:
The root element that rems size is relative to is the html element. The 'rem', which stands for "root em", is a scalable unit in CSS (Cascading Style Sheets) that is used for font sizing and other measurements within a web page. The 'rem' unit is relative to the font-size of the root element, which is the html element. When you define a size in 'rems', it's always in relation to the size of the font that is set on the html element, not any other element. This allows for consistency throughout the website, as all elements sized with 'rems' will scale proportionally.