Using px or pixels in CSS sizing is generally not recommended. This document explores alternative units of measurement that can be employed instead of px. But first, let's delve into why px is often considered a poor choice.
The Problem with Pixels
Everything displayed on a screen comprises pixels. But why does using pixels for measurement pose a problem? Websites are viewed on a variety of screens, each differing in size, orientation, zoom levels, and pixel geometries. What looks perfect on one screen might be distorted on another.
Consider a div on your website set to 600px, occupying half of your screen. When you resize the screen, zoom in or out, or view the site on a mobile device, your website might not display as intended. Moreover, in CSS, a pixel doesn't always correspond to a physical pixel on your screen. On high-resolution displays, several device pixels can combine to form a single CSS pixel.
Additionally, pixel-based sizing scales poorly. For users with poor eyesight who have increased their device's font size, a fixed font size like 14px on your website might render the text unreadable, as it doesn't adjust to their preferred settings.
Alternatives to Pixels
So, what are the alternatives to using px? Units like em, rem, ch, ex, vh, vw, and others offer more flexibility. Let's explore them in detail.
Understanding rem and em
The rem unit is relative to the font size of the root element of the document, which is usually the <html> element. This means that if the font size of the <html> element is set to 16 pixels, 1rem will be equivalent to 16 pixels. Using rem for font sizes, margins, padding, and other properties ensures consistency throughout your website. For example, setting paragraph font sizes to 1rem means that they will scale appropriately if the user has different default font size settings in their browser. This makes rem particularly useful for maintaining accessibility, as it adapts to the preferences of users with poor eyesight.
The em unit is relative to the font size of the element it's used on. It's useful for scaling designs within an element, such as for text, margins, and padding. If em is used for an element's font size, it's relative to its own font size. For other properties, it's based on the element's text size.
Take a look at the example:
<style>
html {
font-size: 16px;
}
h2 {
font-size: 2.5rem;
} /* 40px if root font size is 16px */
h2 span {
font-size: 2em;
} /* Twice the font size of h2 */
</style>
<h2><span>Larger</span> Text</h2>
In this example, changing h2's font size automatically adjusts the <span> size, showcasing em's adaptability.
What Are ch and ex?
Ch and ex units refer to the width and height of a character, respectively. 1ch is the width of the character 0, and 1ex is the height of the x character in your font. For instance, if you want a paragraph with a consistent character count per line regardless of font size, you can use:
<style>
p {
max-inline-size: 30ch;
}
</style>
<p>Long text...</p>
Viewport Units
Viewport units are responsive to the size of the user's browser window. 1vw represents 1% of the viewport's width, and 1vh is 1% of its height. To create a div that takes half of the viewport's width, set its width to 50vw. The same concept applies to vh.
Additionally, vmin and vmax are based on the smaller and larger dimensions of the viewport. A div with width: 50vmin and height: 50vmax occupies 50% of the viewport's smallest width and largest height.
Percentages (%)
Percentages are relative to the size of the parent element. For example:
div {
width: 600px;
}
div p {
width: 50%;
}
In this case, the p tag inside the div will be 300px wide.
Absolute Lengths
CSS also offers units like cm, mm, in, pt, etc., based on physical measurements. However, like px, these units can vary depending on the screen and are typically not recommended.
In conclusion, while pixels may seem straightforward, they often lead to less flexible and accessible designs. Embracing alternative units like em, rem, ch, ex, vw, vh, vmin, vmax, and percentages can make your web designs more adaptable and user-friendly.
Top comments (5)
I only use REM for font-size.
Thanks for your input! Using
remfor font-size is a solid choice for scalability and accessibility.I used to like em, but I felt higher rendering performance when using px.
Although I don't have concrete evidence like benchmarks.
emis a relative size to their parent's font size, not an absolute size, that's perhaps why you notice some slight delay on large pages. Be warry when using it.rem(root em) on the other hand is relative to the base font-size of the HTML page. Recommended.Thanks for the reply @dagnelies 😊 I also agree with you.