PostCSS Blank Pseudo

npm install css-blank-pseudo --save-dev

PostCSS Blank Pseudo lets you style form elements when they are empty, following the Selectors Level 4 specification.

To use this feature you need to do two things :

• add the PostCSS plugin that transforms the selector into a class or attribute
• add the browser polyfill that sets the attribute or class on elements in a browser

input:blank {
	background-color: yellow;
}

/* becomes */

input[blank].js-blank-pseudo, .js-blank-pseudo input[blank] {
	background-color: yellow;
}
input:blank {
	background-color: yellow;
}

Usage

Add PostCSS Blank Pseudo to your project:

npm install postcss css-blank-pseudo --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssBlankPseudo = require('css-blank-pseudo');

postcss([
	postcssBlankPseudo(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

Options

preserve

The preserve option determines whether the original notation is preserved. By default, it is preserved.

postcssBlankPseudo({ preserve: false })

input:blank {
	background-color: yellow;
}

/* becomes */

input[blank].js-blank-pseudo, .js-blank-pseudo input[blank] {
	background-color: yellow;
}

replaceWith

The replaceWith option determines the selector to use when replacing the :blank pseudo. By default is [blank]

postcssBlankPseudo({ replaceWith: '.css-blank' })

input:blank {
	background-color: yellow;
}

/* becomes */

.foo {
	color: blue;
	color: red;
}

.baz {
	color: green;
}

Note that changing this option implies that it needs to be passed to the browser polyfill as well.

disablePolyfillReadyClass

The disablePolyfillReadyClass option determines if selectors are prefixed with an indicator class. This class is only set on your document if the polyfill loads and is needed.

By default this option is false. Set this to true to prevent the class from being added.

postcssBlankPseudo({ disablePolyfillReadyClass: true })

input:blank {
	background-color: yellow;
}

/* becomes */

input[blank] {
	background-color: yellow;
}
input:blank {
	background-color: yellow;
}

Browser

import cssBlankPseudoInit from 'css-blank-pseudo/browser';

cssBlankPseudoInit();

or

<!-- When using a CDN url you will have to manually update the version number -->
<script src="https://unpkg.com/[email protected]/dist/browser-global.js"></script>
<script>cssBlankPseudoInit()</script>

PostCSS Blank Pseudo works in all major browsers, including Safari 6+ and Internet Explorer 9+ without any additional polyfills.

This plugin conditionally uses MutationObserver to ensure recently inserted inputs get correct styling upon insertion. If you intend to rely on that behaviour for browsers that do not support MutationObserver, you have two options:

1. Polyfill MutationObserver. As long as it runs before cssBlankPseudoInit, the polyfill will work.
2. If you don't want to polyfill MutationObserver you can also manually fire a change event upon insertion so they're automatically inspected by the polyfill.

Browser Usage

force

The force option determines whether the library runs even if the browser supports the selector or not. By default, it won't run if the browser does support the selector.

cssBlankPseudoInit({ force: true });

replaceWith

Similar to the option for the PostCSS Plugin, replaceWith determines the attribute or class to apply to an element when it's considered to be :blank.

cssBlankPseudoInit({ replaceWith: '.css-blank' });

This option should be used if it was changed at PostCSS configuration level. Please note that using a class, leverages classList under the hood which might not be supported on some old browsers such as IE9, so you may need to polyfill classList in those cases.

Using with Next.js

Given that Next.js imports packages both on the browser and on the server, you need to make sure that the package is only imported on the browser.

As outlined in the Next.js documentation, you need to load the package with a dynamic import:

useEffect(async () => {
	const cssBlankPseudoInit = (await import('css-blank-pseudo/browser')).default;
	cssBlankPseudoInit();
}, []);

We recommend you load the polyfill as high up on your Next application as possible, such as your pages/_app.ts file.