EM (short for "Ems") is a unit of measurement in web design that is used to specify font sizes and spacing. It is a relative unit that depends on the size of the font in the parent element.

The size of an EM is defined by the font size of the parent element. For example, if the font size of the parent element is 16 pixels, then 1 EM corresponds to the size of 16 pixels. If the font size of the parent element is changed, the size of EMs defined within this element also changes.

The advantage of EMs in web design is that they are more flexible than absolute units such as pixels. If a font size is defined in EMs, the size automatically adapts to the font size of the parent element. This is particularly useful when a website needs to be optimised for different screen sizes and devices.

In neuroweb design, the use of EMs can help to ensure that the text size and line spacing on a website are optimised and that the readability and comprehensibility of the content is improved. Careful use of EMs can therefore help to improve the user experience and usability.