Understanding CSS Units

px (Pixel): An absolute unit of measurement. One pixel is a single dot on the screen. Pixels are great for things that should not scale, like borders, but can cause accessibility issues if used for font sizes.
rem (Root EM): A relative unit based on the font size of the root element (the <html> tag). By default, this is 16px in most browsers. Using REMs allows all elements on your page to scale up or down uniformly if a user changes their browser's default font size, which is great for accessibility.
em: A relative unit based on the font size of its parent element. This can be powerful but can also lead to compounding and unexpected results if elements are nested deeply. REMs are often easier to manage.

Why Use REMs for Sizing?

Using rem units for fonts, padding, and margins ensures your website is scalable and accessible. Users who need larger text for readability can adjust their browser's default font size, and a REM-based design will scale proportionally, maintaining a consistent and usable layout.