While it's best practice to use em or rem when defining font sizes, it's common for developers to be unsure of which one to use or even ho...
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Coming from a typography background,
ems (actually mostlyrems) come naturally. Knowing the history behind it — the width of the box that holds the movable type letterM, which in most classic typefaces used to be to be the widest character — helps to understand the meaning behind it. Some other useful em-related characters include the em space (a space as large as a typographical em), can be accessed with , and the em dash (—), which is used to separate ideas, similarly to parentheses. On Mac, you can type it using option+shift+hyphen: —.Fun fact: Similarly to
ems, there are alsoens. Like the em space, there's also an en space ( ), and an en dash (–), achieved with option+hyphen. It is technically measured as the width of half an em, though in some typefaces' designs the en dash width isn't the right width. Bonus fun fact, an en dash is the correct typographic symbol to use when referring to a minus (as in 4–1=3), however, if you use it in programming, it's not going to work. The hyphen, what we normally use as a minus symbol, is just a hyphen, the symbol used at the end of a line to separate a word when it doesn't fit in the line (called hyphenation). The en dash is properly used in text in some cases, like when denoting time ranges (2017–2019).Back from typography nerd talk... I personally use
rems 99% of time in CSS because it's most predictable. I set it to be 16px in my html or body CSS declaration, and then I know I can work in multiples or divisions of 16. I even have a handy chart written on the wall.Very interesting! Thanks for sharing!
Probably because I learnt most of my CSS long before rem was invented, I tend to reach naturally for em and only use rem when I specifically need it. I think rem is naturally easier for beginners to understand and fits better into a twitter-bootstrap or BEM organised way of styling, where the structure of the DOM document is downplayed as much as possible.
But if you believe in exploiting the richness of selectors and the structure of the DOM in your styling, then the em unit comes into its own. For example suppose you have some auxiliary content to the main content of your page. On a desktop, you might decide to present this in a sidebar. Because it's auxiliary content on a large pixel width display, you want to use a smaller font for the contents of the sidebar than the main content, so the eye isn't attracted to it so much. So you use em for all the descendant elements of the auxiliary content container, which are therefore ultimately all relative to the font-size defined for that auxiliary content container.
Then, for the rendering on a mobile, you decide that a sidebar doesn't work, and instead the auxiliary content should be displayed below the main content. In this case, you decide that don't want to use a smaller font than the main content. All you need to change for your font size in your media query is the font-size of the auxiliary content container, and everything will adjust itself appropriately. If you had used rem instead, that wouldn't have been possible.
Very good point. If I were working on a project of my own, I'd probably be more inclined to use
emfor things of that nature. Thanks for sharing that better use case forem!This post actually clarified the differences a lot for me. A few audible "oh!"'s were heard (I don't know if I punctuated that correctly). Anyway, thank you for the quick and educational read!
Awesome! Glad to hear it helped you! I know this was a gap in my own knowledge for a loooong time so I’m glad I was able to put it together and share!
Great post!
I think
remis a better choice for a lot of stuff, but I haven't found myself using it just because I have the habit of usingem.Old habits die hard.
I can absolutely relate! 😅
Thanks for the great explanation!
You're very welcome! I'm glad I could help!!
i learned from a course on udemy that when set font-size 62.5% in html tag, using rem easier to control, what do you think about this ?
It just makes the math easier. As long as you're not setting the font size to 10px explicitly (as this has negative accessibility impacts), it's just personal preference. I don't mind working with 16px, so I just prefer not to change it. 😊