Why would I ever NOT use percentage for sizes?

Question

I am an embedded programmer (no GUI), who codes Angular, and, occasionally, Flutter, for fun.

In general, my apps work, but look like an early 90s GeoCities site :-)

On thing that I just can't get my head around - and could someone please explain it like I am five years old - is why I would ever want to use any unit of measurement other than percentage.

Given the number of different devices out there, the new ones appearing daily, and as yet imagined devices, plus, of course, just plain resizing the browser window, it just doesn't seem to make sense to me.

The Mozilla documentation offers me a staggering choice of

• cm
• mm
• Q
• in
• pc
• pt
• px

which are all fixed size.

I can also use

• em
• ex
• ch
• rem
• lh

which are effectively fixed (with reference to their ratio to a given item), and only make sense with text (can they even be applied to other elements?).

I like the look of

• vw
• vh
• vmin
• vmax

which are percentages of the viewport, plus of course plain old height: 50%, etc

I am aware of responsive design, but that seems to me to be saying that there are X different screen sizes, then proceeding to use px on each of them, having the same scaling problems on X different screen sizes.

It seems intuitive to me that percentages are the only truly proportional way to define resize-proof sizes, but it can't be that simple, because every book, tutorial, Udemy curse that I look at seems to use px, or just about anything other than %.

So, please explain it to me like I am five. Why not use % or viewport related sizes on everything?

---

[Update] text is the least of my concerns. I am more interested in the relative sizes of every element. Of course, I can use flex grid, etc. But even a simple border-radius is not going to scale very will if it is defined in px. I want my page to look identical at all/any resolution(s).

Answer 1

I, like you and many other people starting to learn CSS, used to think the same thing. However, as I continued to build websites I realized that using percents only leads to uglier websites. There are a few reasons that contribute to this:

1. Raster type, images, etc. don't scale perfectly. The browser has to approximate the rendering of these elements when they don't perfectly line up with the pixels of the screen. Browsers do an alright job of this most the time but there are a lot of cases where they don't and using percents on everything will pretty much guarantee poor rendering on parts of your page for at least some viewport sizes.

2. Lack of consistent white space. It's almost always best to use a unit based on the font or pixel size (like px, em, or rem) for margins and padding because otherwise you'll have inconsistent white space even when the font size is being changed. This is especially noticeable when the margin or padding is smaller because the white space rendering being 1 pixel off is more noticeable.

3. It's harder to maintain. Percents are based on the inherited value, which means that if you have any real amount of nesting of elements (and most websites have a lot) you will have some style rules that are in place to override the inherited value because otherwise the sizing of elements will be inconsistent depending on how an element is nested. This the reason why the rem unit exists.

4. It's lazy programming. If your job is to make a responsive website, it's easy to slap some percentages in there and call it "responsive" when in reality good responsive design will make use of all of the tools (including percentages at times) available to make an optimal website for every used viewport sized.

The units in CSS are all there for a reason. In general the physical world units (cm, in, etc.) aren't used for web development but the rest are all used regularly when you've gotten to a point of being pretty comfortable with them.

The most common units that I use are rem, em (most type should use these first two units if possible), px, %, vw, vh, vmin, and vmax.

Answer 2

Percentages are sometimes too flexible. If you define everything in percentages then the site will scale uniformly. But its very unlikely for a user of a 27-42 inch monitor wants you to scale text by their screen width. Same way a small screen user may want the font to stay relatively fixed when they turn the phone.

So you want to have some items at fixed sizes. Since your font has a more fixed size then you want sidebars and some of the whitespace fixed and so on. Because a sidebar that is smaller than the text makes little sense.

Then there are the images...

Answer 3

Pixel art and other items designed as pixel perfect become blurry as soon as they are not shown in the right pixel size or its multiple.

The problem would vanish if every visible item was designed as freely scalable vectors. The display resolutions should be so high that the necessary antialiasing, when vectors are rendered would not be seen as blurriness.

But they are not. The usual screen resolution as pixels per inch is far too low. For ex. texts on the screen are vectors and to make them look sharp every character must be placed separately to fit as well as possible into the pixel grid. The final smoothing is done by usual anti-aliasing (a kind of blurring), but it's not needed much if the fine placements of the characters are hinted properly. Every shown small item should be placed with the same care if one expects the same sharpness and consistency as we already have with texts. Having pixel perfect items is simpler.

Answer 4

I am aware of responsive design, but that seems to me to be saying that there are X different screen sizes, then proceeding to use px on each of them, having the same scaling problems on X different screen sizes.

There is some truth in there. Some people, when doing responsive versions of a site (be it mobile-first or desktop-first), will have common breakpoints in px from whatever they were trained on (768, 1024, etc.). I wouldn't say that this is right or wrong, it is just what some people are used to. Other people will tell you to use em or rem for these. If you dive deep enough, however, most people tend to point to this article from 5+ years ago with a Safari bug as to why, and I haven't seen anyone post a more recent test (nor have I even tried, and I'm an em guy).

which are effectively fixed (with reference to their ratio to a given item), and only make sense with text (can they even be applied to other elements?).

You can absolutely use them with more than just text. For instance, if you have a button you might want the space in the inline direction (left and right in the traditional syntax) to be relative to the current font size, so something like padding-inline: 2em might make sense for a specific design.

It seems intuitive to me that percentages are the only truly proportional way to define resize-proof sizes, but it can't be that simple, because every book, tutorial, Udemy curse that I look at seems to use px, or just about anything other than %.

I think the major problem with percentages is that the question always comes down to "percentage to what?". To the viewport? To the box? To the current font size? And if we are a percentage of something, is that something a percentage of something else. This is actually why you'll often see people using rem over em, just because the calculation is more straightforward and obvious, and doesn't change as you nest things. Also, percentages in the block direction (traditionally top and bottom) can be vague, or they at least not as understood.

So, please explain it to me like I am five. Why not use % or viewport related sizes on everything?

We used to do that, actually, at least with percentages. It was very common to do a 35%/65% two column layout, although in reality because of whitespace or padding/margin issues we'd cheat and do something like 33%/62% or something. Eventually some of us figured out the font-size: 0 trick on the parent, or we'd do the </div><!-- vertical whitespace -><div> trick, or possibly just force no whitespace such as </div><div>, but it always felt hacky and it was very fragile, especially once we gave up control to CMSs.

So that's where grid and flexbox came in. We knew we wanted "percentage based layouts", but the rules for percentages were already set in stone and changing them would be a giant backwards compatibility break.

As to why not viewports, they suffer from the same problem as percentages, at least as far as "two things next to each other" where you have to hack to make it work. I do think that viewport units do shine, however, very specifically when used as a min or maximum size. For instance, for a font in a header you could clamp it to a maximum of 5vw which allows you to scale, but only up to a certain point. Same with a box. At a minimum it should be 1/3 of the screen, but if there's room it can grow bigger. But they also suffer from the fact that 50vw on my mobile device if drastically different than my landscape desktop setup.

I think my summary would be:

The rules for percentages were set in stone a long time ago and can't be changed. We have a feeling for what we want them to do but it doesn't align with reality. Newer units were created to solve this, however. As to viewports, they have a time and place, but there are so many physical sizes available that you are solving everyone's problems by solving no one's problems.

Edit

Is there a general consensus of best practice? I.e some book, website, Udemy course which will tell me what too use which measurement unit (and why)?

All I can really tell you is what I do, which is based on 20+ years of experience, along with what I'm still learning as I watch other people build things, or inherit other peoples sites.

• For text, use em or rem. They are both the same as long as you understand what they are relative to. Personally, I'm rem for 99.999% of text with maybe a em if I'm doing something funky in ::before or ::after. Also, I still set font-size: 62.5% on the HTML tag although I don't know if that's still cool. If you want fluid text, which not every design calls for, consider using clamp() with vw. I personally don't have many designs that need this, however.
• For margin/padding of text, use em or rem. Especially for paragraphs, you usually want the vertical distance between two text blocks to be essentially "a blank line" which you can get from em or rem.
• For box alignment at a high level, use grid and/or flexbox. I'm personally a grid guy and I use the fr units all the time. If you aren't familiar with fr, they are effectively what everyone wanted percentages to be. Flex is great, too. The when and why really depends on your skill but they honestly can swap in an out 75% of the time.
• For anything else related to boxes, use rem.
• For border radius, percentages are okay, although em and rem might work, too. Just test and see if they scale how you expect.
• For media queries use em, rem or px. I personally use em across the board but like I mentioned above, I haven't re-tested that in years. Another developer on my team uses px, however, and we get along just fine.

That's about it. You'll see I'm rem heavy in my use, but that's just me. 90% of the other units don't interest me, or they are for weird edge cases. For instance, ch can be used to limit line length by character count. If that works for your design, try it, but I still use rem for that, too.

Lastly, one of the biggest hurdles to overcome is to just accept that not everything has to look perfect. Beyond the millions of screen sizes, users can change their zoom as well as their default font size. This is a user explicitly saying that they want something different. The information conveyed needs to be the same, but it can absolutely not look the same as designed. It should be consistent, and you will find edge cases (like line length when you always assumed a header would be a single line how it flows to more lines at certain zooms) and you can tweak as necessary.

Answer 5

I have a laptop with a 13" screen, with 1900 pixel horizontal resolution. I also have a 21" monitor with 1900 pixel horizontal resolution.

I want to be able to read text and see pictures clearly on both of them, so that it's not stupidly small on one or stupidly large on the other. Using a fixed fraction of the screen, or a fixed number of pixels, are both clearly going to fail on one or the other. I hope this is so obvious as to need no explanation. :)

Using a fixed number of centimetres would be better. However then you run into the problem of resolution. On a high-resolution screen, you may be able to read text or see images clearly when they're fairly small; but on a lower-resolution screen that text or image is just going to turn into jagged blocks. On a lower-resolution screen then, I need my text to be a bit physically larger so that it's clear.

This is where "ems" come in. You can set how big text needs to be for any given screen (you may have noticed that you can change this for each screen in display settings). This can then be used as a guide for how large everything else needs to be in order to be displayed clearly.

Of course some things don't necessarily scale - borders, bitmap images and so on. In those cases you need fixed sizes.

Where would you use percent? Most obviously, any time you need your screen divided into fixed fractions of the area. If I'm comparing two files side by side, I probably want each to take 50% of the screen width.

Horses for courses, in other words. There are use cases for all of them. As the saying goes, the best hammer makes a very poor screwdriver. Learn which jobs need which.