The rem, or root em, is the font size of the root element of the document. When user sets different font-size on browser, REM automatically scale up and down elements like fonts, images etc on the webpage which is not the case with PX. Broadly speaking, there are two categories of units, absolute and relative. In other words, if you have the following CSS: .element { font-size: 20px; }. REM Stands For: All acronyms (266) Business & Finance (12) Common (3) Government & Military (25) Medicine & Science (43) Chat & Sub Cultures (1) Education Schools (6) Technology, IT etc. To recap, the em unit means "my parent element's font-size". In HTML4, this is always the element, since it is the highest-level ancestor of all other elements on the page. Ah-em — Definition Please The spec gives us a very simple definition for the em unit: Equal to the computed value of the 'font-size' property of the element on which it is used. Meanwhile, rem is defined as the font-size of the root element. A rem is equal to 0.01 sievert in the International System of Units (SI). In CSS specifications and on the property pages here on MDN you will be able to spot values as they will be surrounded by angle brackets, such as `s get reset. 'Removal' is one option -- get in to view more @ The Web's largest and most authoritative acronyms and abbreviations resource. The em unit is relative to the font-size of the parent, which causes the compounding issue. Very handy when doing full screen slides… It also has good support: http://caniuse.com/#feat=viewport-units. Theyâre actually optical reference units . Not all fonts have the letter M in them (for example, Chinese), but all fonts have a height. Numeric Values â Length has a number followed by a unit like 10px, 5mm, 8in etc. The rem unit, short for root em is a relative unit thatâll always be based upon the font-size value of the root element, which is the element. Simply so, how do you use REM units in CSS? What does a positive biuret test look like? What Dan says is 100% correct, but a liitle background info about emâs. All the latest browsers suppot ‘rem’ unit. CSS3 now has rem (“root em”) to prevent that issue. Copyright @ cssstars.com 2015, RouterLinkActive sets active class to Multiple Links. EM is scalable, but in CSS3 ther is a new unit called ‘REM’. So by default 1em = 16px. Viewport Width (vw) — This unit is based on the width of the viewport. If the browser default font-size is 16px, 62.5% of 16 is 10px. What is the fallback option?? The rem unit (or âroot emâ unit) has pretty good browser support: IE9+, FF3.6+, Chrome, Safari 5+, and Opera 11.6+. Also Know, what is 100vh in CSS? CSS rem unit to scale specific page elements while leaving others unaffected. showing only Science & Medicine definitions . The 'rem' Now onto the rem unit. 12px = 0.75rem. Looking for the definition of REM? In this context REM and EM in CSS get its prominance. However, we also reset the bottom margin to use `rem` units instead of `em`. New Unit (rems) for Typography Typography font sizing in Bootstrap 4 is done completely based on the rem unit of measurement. Let's see an example of some common font sizes expressed in rem units, assuming, of course, that the base size is 16px: 10px = 0.625rem. Let's look at all the different kinds of selectors available, with a brief description of each. While most browsers default the html font size at 16px (meaning 1 rem = 16 pixels), we can adjust the root font size in order to achieve our desired outcome.