Importing Styles Into A Web Component
Solution 1:
Now direct <link> tag is supported in shadow dom.
One can directly use:
<link rel="stylesheet" href="yourcss1.css">
<link href="yourcss2.css" rel="stylesheet"type="text/css">
It has been approved by both whatwg and W3C.
Useful links for using css in shadow dom:
- https://w3c.github.io/webcomponents/spec/shadow/#inertness-of-html-elements-in-a-shadow-tree
- https://github.com/whatwg/html/commit/43c57866c2bbc20dc0deb15a721a28cbaad2140c
- https://github.com/w3c/webcomponents/issues/628
Direct css link can be used in shadow dom.
Solution 2:
If you need to place external styles inside the <template> tag you could try
<style>@import"../my/path/style.css"; </style>however I have a feeling this will start importing after the element has been created.
Solution 3:
Constructable Stylesheets
This is a new feature that allows for the construction of CSSStyleSheet objects. These can have their contents set or imported from a css file using JavaScript and be applied to both documents and web components' shadow roots. It will be available in Chrome with version 73 and probably in the near future for Firefox.
There's a good writeup on the Google developers site but I'll summarize it briefly below with an example at the bottom.
Creating a style sheet
You create a new sheet by calling the constructor:
const sheet = newCSSStyleSheet();
Setting and replacing the style:
A style can be applied by calling the methods replace or replaceSync.
replaceSyncis synchronous, and can't use any external resources:sheet.replaceSync(`.redText { color: red }`);replaceis asynchronous and can accept@importstatements referencing external resources. Note thatreplacereturns aPromisewhich needs to be handled accordingly.sheet.replace('@import url("myStyle.css")') .then(sheet => { console.log('Styles loaded successfully'); }) .catch(err => { console.error('Failed to load:', err); });
Applying the style to a document or shadow DOM
The style can be applied by setting the adoptedStyleSheets attribute of either the document or a shadow DOM.
document.adoptedStyleSheets = [sheet]
The array in adoptedStyleSheets is frozen and can't be mutated with push(), but you can concatenate by combining with its existing value:
document.adoptedStyleSheets = [...document.adoptedStyleSheets, sheet];Inheriting from the document
A shadow DOM can inherit constructed styles from the document's adoptedStyleSheets in the same way:
// in the custom element class:this.shadowRoot.adoptedStyleSheets = [...document.adoptedStyleSheets, myCustomSheet];
Note that if this is run in the constructor, the component will only inherit the style sheets that were adopted prior to its creation. Setting adoptedStyleSheets in the connectedCallback will inherit for each instance when it is connected. Notably, this will not cause an FOUC.
Example with Web Components
Let's create a component called x-card that wraps text in a nicely styled div.
// Create the component inside of an IIFE
(function() {
// template used for improved performanceconst template = document.createElement('template');
template.innerHTML = `
<div id='card'></div>
`;
// create the stylesheetconst sheet = newCSSStyleSheet();
// set its contents by referencing a file
sheet.replace('@import url("xCardStyle.css")')
.then(sheet => {
console.log('Styles loaded successfully');
})
.catch(err => {
console.error('Failed to load:', err);
});
customElements.define('x-card', classextendsHTMLElement {
constructor() {
super();
this.attachShadow({
mode: 'open'
});
// apply the HTML template to the shadow DOMthis.shadowRoot.appendChild(
template.content.cloneNode(true)
);
// apply the stylesheet to the shadow DOMthis.shadowRoot.adoptedStyleSheets = [sheet];
}
connectedCallback() {
const card = this.shadowRoot.getElementById('card');
card.textContent = this.textContent;
}
});
})();<x-card>Example Text</x-card><x-card>More Text</x-card>Solution 4:
NB!!!
THIS ANSWER IS OUTDATED
PLEASE CHECK THE ANSWER BY Himanshu Sharma below
Up-to-date answer: https://stackoverflow.com/a/48202206/2035262
According to Polymer documentation:
Polymer allows you to include stylesheets in your<polymer-element>definitions, a feature not supported natively by Shadow DOM.
This is a bit weird reference, but I could not google the straight one. It looks like at the moment there is no rumors about supporting links inside templates.
That said, whether you want to use vanilla web component, you should either inline your css with <style> tag, or load and apply your css manually in javascript.
Solution 5:
The above answers show how to import stylesheets into a web component, but importing a single style to a shadow DOM can be done (kind-of) programmatically. This is the technique I developed recently.
First - make sure that you embed your component-local styles directly in a template with the HTML code. This is to make sure that the shadow DOM will have a stylesheet in your element constructor. (importing other stylesheets should be ok, but you must have one ready in the constructor)
Second - use a css-variable to point at a css rule to import.
#rule-to-import {
background-color: #ffff00;
}
my-element {
--my-import: #rule-to-import;
}
Third - In the component constructor, read the CSS variable and locate the pointed to style in the document stylesheets. When found, copy the string but rewrite the selector to match the internal element(s) you wish to style. I use a helper function for this.
importVarStyle(shadow,cssvar,target) {
// Get the value of the specified CSS variableconst varstyle=getComputedStyle(this).getPropertyValue(cssvar).trim();
if(varstyle!="") varstyle: {
const ownstyle=shadow.styleSheets[0];
for(let ssheet ofdocument.styleSheets) { // Walk through all CSS rules looking for a matching rulefor(let cssrule of ssheet.cssRules) {
if(cssrule.selectorText==varstyle) { // If a match is found, re-target and clone the rule into the component-local stylesheet
ownstyle.insertRule(
cssrule.cssText.replace(/^[^{]*/,target),
ownstyle.cssRules.length
);
break varstyle;
}
}
}
}
}
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