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HTML 

              
                <h1>Minimalist Container Queries</h1>

<p>By <a href="https://scottjehl.com">Scott Jehl</a></p>

<p>This <code>c-q</code> custom element enables very simple "min-width" container queries, allowing you to adjust its CSS based on its own rendered width instead of viewport width.</p>

<h2>Example:</h2>


<c-q>
  <p>resize me!!</p>
</c-q>

<h2>Usage</h2>

<p>To use, set a given <code>c-q</code>'s <code>--breakpoints;</code> custom CSS property to one or more pixel-based breakpoints that the JS should be aware of, like this:</p>
<pre><code>c-q { --breakpoints: "400 600 800"; }</code></pre>
<p>One or more of these values will appear in the element's <code>data-min-width</code> attribute when they are greater or equal to the element's rendered width. You can style the element based on their presence using the <code>~=</code> attribute selector, like this: </p>
<pre><code>c-q[data-min-width~="400"] { background: green; }</code>. </p></pre>




<style>
/* demo styles */
  body {
  font-family: sans-serif;
  margin: 3vw;
}
code {
  padding: .3em .5em;
  font-size: 1em;
  background: #333;
  color: #fff;
  border-radius: .2em;
}
p {
  max-width: 45em;
  line-height: 1.5;
}
c-q {
  resize: both;
  display: block;
  overflow: auto;
  height: 50px;
  width: 300px;
  padding: 10px;
}
</style>
!

CSS 

              
                c-q {
  --breakpoints: "400 600 800";
  background: tan;
}
c-q[data-min-width~="400"] {
  background: green;
}
c-q[data-min-width~="600"] {
  background: blue;
}
c-q[data-min-width~="800"]  {
  background: red;
}
!

JS 

              
                // Minimalist Container Queries
// 2020 @scottjehl, @filamentgroup / MIT License
class CQ extends HTMLElement {
  constructor() {
    super();
    this._init = this._init.bind(this);
    this._observer = new MutationObserver(this._init);
  }
  connectedCallback() {
    if (this.children.length) {
      this._init();
    }
    this._observer.observe(this, { childList: true });
  }
  _init() {
    var self = this;
    this._rObserver = new ResizeObserver((entries) => {
      for (const entry of entries) {
        self.setActiveBPs(entry);
      }
    });
    this._rObserver.observe(this);
  }

  setActiveBPs(entry) {
    var bps = window.getComputedStyle(entry.target).getPropertyValue("--breakpoints");
    var activeBPs = [];
    if (bps) {
      bps = bps.replace('"', "");
      bps.split(" ").forEach((bp) => {
        var bpnum = parseFloat(bp);
        if (entry.contentRect.width >= bpnum) {
          activeBPs.push(bpnum);
        }
      });
    }
    entry.target.setAttribute("data-min-width", activeBPs.join(" "));
  }
}
customElements.define("c-q", CQ);
!
999px

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