Frontend performance and user experience are orthogonal to feature development. If care is not taken, adding features usually degrades frontend performance over time.

Many years, frontend user experience has been hard to quantify. However, Google has developed some metrics to capture user experience on the web: the Web Vitals. The Core Web Vitals are about "felt loading speed" (Largest Contentful Paint), reactivity (Interaction to Next Paint) and visual stability (Content Layout Shift).

I have recently be...

Modern CSS offers the field-sizing property to allow elements to automatically adjust size (width and/or height) to fit their contents.
The most common use case are textareas which start fairly small (e.g. 2 or 3 rows tall) but grow when users enter longer text.

Usage

textarea {
  field-sizing: content;
}

That's it! At least in modern Chromium-based browsers.

Limited browser support

Support is still lacking in Firefox and Safari (as of mid 2025)....

Unpoly 3.11.0 is a big release, shipping many features and quality-of-life improvements requested by the community. Highlights include a complete overhaul of history handling, form state switching and the preservation of [up-keep] elements. We also reworked major parts of the documentation and stabilized 89 experimental features.

We had to make some breaking changes...

We usually ship applications that self-host webfonts to comply with GDPR.

Many popular web fonts are available as NPM packages provided by Fontsource.
We recommend using those instead of downloading and bundling font files yourself. (See below for a list of benefits.)

Usage

1. Go to fontsource.org and search for the font you want to add (or a font that suits your application).
2. Click the font card to vie...

Use these snippets when you want to measure yourself.

Currently available:

Core Web Vitals

Largest Contentful Paint (LCP)
Largest Contentful Paint Sub-Parts (LCP)
Quick BPP (image entropy) check
Cumulative Layout Shift (CLS)

Loading

Time To First Byte
Scripts Loading
Resources hints
Find Above The Fold Lazy Loaded Images
Find non Lazy Loaded Images outside of the viewport
Find render-blocking resources
Image Info
Fonts Preloaded, Loaded, and Used Above The Fold
First And Third Party Script Info
First And Third Party Script Timings
I...

It can be hard to understand what causes a browser scroll smoothly or instantly. CSS, JavaScript and the browser settings all have options to influence the scroll behavior. They all interact with each other and sometimes use the same words for different behavior.

CSS

Scrolling elements can use the scroll-behavior CSS property to express a preference between smooth and instant scrolling.

Preferring instant scrolling

CSS can prefer instant scrolling behavior:

html {
  scroll-behavior: auto; /* the default */
}

An `aut...

To apply transparency to an element, you can use opacity in CSS. However, sometimes you don't want to make the entire element transparent, only a color.
Using not-so-modern CSS, you can simply generate non-opaque versions of a color. This card describes how to do that.

Note that we're not talking about defining colors with transparency. This is about the case where you have a color but need a more transparent variant of it (e.g. for a border, background, etc.), and where your component does that without defining the transparent variant ...

The currentColor CSS keyword references the current text color and can be used to apply an element's text color to other properties.
Using modern CSS, you can also use it to calculate colors from it. This card describes how to invert currentColor, but you can use this technique for other calculations as well.

How to invert currentColor

They key is CSS' hsl() function and from keyword for it:

--inverted-color: hsl(from ...

The :nth-child pseudo class is commonly used for targeting elements based on their position within a parent container, for example the third element in a list or every even-numbered item. However, :nth-child can do more.

In modern CSS specifications (Level 4), there’s an additional feature that lets you use :nth-child in combination with a list of css selectors. This way, you can ta...

DirectUpload allows you to upload files to your file storage without having to wait for the form to submit. It creates AJAX requests to persist the file within your form and wraps them in a little API. This card will show you how to use it in order to create in-place file uploads with progress and a remove button.

This is basic functionality, you may add additional elements, styles and logic to make this look fancy, but the core functionality is the same. I created a file upload that looks like this:

![Image](/makandra/625023/attachments/3...

This decision tree describes how to use the alt attribute of the <img> element in various situations. For some types of images, there are alternative approaches, such as using CSS background images for decorative images or web fonts instead of images of text.

Questions asked:

• Does the image contain text?
• Is the image used in a link or a button, and would it be hard or impossible to understand what the link or the button does, if the image wasn’t there?
• Does the image contri...

Elements can be hidden and shown by toggling the display property. However, this is not animatable, so we often turn to opacity. At opacity: 0, the element is hidden, and with a nice transition on that property, it can be faded in and out smoothly.

Yet, opacity only hides visually, not technically: the element is still focusable and visible to screen readers. So, how can we fade an element while maintaining accessibility?

Enter visibility. It also hides elements, bu...

You are given two CSS selectors that you do not control. How can you build a new selector that matches both of them?

item_selector = 'div'
active_selector = '.is-active'

Can't I just concat these selectors?

# Bad
new_selector = "#{item_selector}#{active_selector}"
# => "div.is-active"

Don't! This will break as soon as one of the selectors is actually a selector list.

item_selector = 'div, span, p' # <- Selector list 
new_selector # => "div, span, p.is-active" (wrong)

Solution

Wrap both selectors ...

Modern JavaScript includes Intl.NumberFormat to format numbers in different formats and locales.
In this card, we describe a wrapper for it that humanizes a given number of seconds in the "next best" unit, like seconds, minutes, etc.

Example usage

>> new Duration(42).humanized()
=> '42 Sekunden'
>> new Duration(123456).humanized()
=> '1 Tag'
>> new Duration(123456).humanized('es')
=> '1 día'

Code

Here is the code ...

tl;dr When browsers start to adapt proposals from Open UI, it might not be necessary to use any 3rd party libraries to have nice components and controls in web applications e.g. selects. It would require only a minimum of CSS and Javascript to get them working and looking good.

The purpose of the Open UI, a W3C Community Group, is to allow web developers to style and extend built-in web UI components and controls, such as <select> dropdowns, checkboxes, radio buttons, and date/color pickers.

To do that, we’ll need to fully speci...

Note

Using rem only ever makes sense when the root font size is dynamic, i.e. you leave control to the user. Either by a) respecting their user agent defaults, or by b) offering multiple root font sizes in your application.

By defining @media queries in rem, they will accommodate to the root font size of your page. At a larger root font, breakpoints will be at larger widths, scaling with the font. However, there is a catch in case b) mentioned in the note above.

Relative length units in media queries are based on the initial value,...

Rachel Andrew has built a website about CSS Grid.

• Video tutorials
• More than 30 layout examples for feature demonstration
• Layout patterns for copy-paste use
• All grouped by topic: "Placing items onto the grid", "Sizing of tracks and items" etc. with video, linked articles, examples each

These are the top ten accessibility errors as researched by TPGi, a company focusing on accessibility. See the linked article for details on each item, as well as instructions on how to do it correctly.

1. No link text
2. Non-active element in tab order
3. Missing link alt attribute
4. No alt text
5. List not nested correctly
6. Duplicate labels used
7. Positive tabindex value
8. Invalid aria-describedby
9. No label for button element
10. Invalid aria-labelledby

Generally, I am surprised by these items. I would have expected more complex i...

The Web Animations API has great browser support, and you should be using it to animate DOM elements from JavaScript, or to control or wait for CSS animations.

Here is a quick overview of a few useful features:

Animating elements from JavaScript

Use the Element#animate() function to perform animations on an element.

Its API probably a bit different from how your...

Up until Chrome 120, scrollbars could only be styled using the various -webkit-scrollbar-* pseudo elements, e.g. to make the scrollbars have no arrows, be rounded, or with additional margin towards their container.

Starting with version 121, Chrome now also supports the spec-compliant properties scrollbar-width and scrollbar-color.
These allow less styling. You may only specify the track and thumb colors, and a non-specific width like auto, thin, or none.

Adding a unit

Multiply by 1x the unit:

$number = 13
$length = $number * 1px // => 13px

Removing a unit

Divide by 1x the unit:

$length = 13px
$number = $length / 1px // => 13

Converting a unit

the result of an addition or subtraction between two numbers of different units is expressed in the first member’s unit

Thus, to convert a number, add it to 0 of the desired unit:

$duration: .21s
$duration-in-milliseconds: 0ms + $duration // => 210ms

An example is storing a transition duration as CS...

If you have an element with significant border-radius (e.g. 50% for a circle) and you want inline content (i.e. text) to flow around it, do it like this:

• Place the element right before the text and float: right or float: left
• Tell the browser to take the content-box for the element's shape, i.e. without margin, padding and border. shape-outside: content-box
• Set the margin where you want it, e.g. 10px left and bottom
• Set the shape-margin to the same size ...

Getting CSS (and JS) live reloading to work in a esbuild / Rails project is a bit of a hassle, but the following seems to work decently well.

We assume that you already use a standard "esbuild in Rails" setup, and have an esbuild watcher running that picks up your source code in app/assets and compiles to public/assets; if not change the paths below accordingly.

Basic idea

We will

• use the guard-livereload gem as the livereload server (which send updates to the browser),
• use the livereload-js npm package in the browser to con...

Even when you app has no CSS at all, you still inherit a default user agent stylesheet from your browser.

These default styles vary from browser to browser:

• Chrome
• Firefox
• Safari

Links found in A look at CSS Resets in 2018.