size-break

While we wait for enough browsers to implement container-queries, and in any event when you simply want to do different things at different sizes (e.g. in the project I'm working on right now, a row of buttons turns into a menu at narrow widths) there's <tosi-sizebreak>.

Note that the sizes referred to are of the <tosi-sizebreak>'s .offsetParent, and it watches for the window's resize events and its own (via ResizeObserver).

<div class="container">
  <tosi-sizebreak min-width="300" min-height="150">
    <h1>BIG!</h1>
    <i slot="small">little</i>
  </tosi-sizebreak>
  <tosi-sizer></tosi-sizer>
</div>

.preview {
  touch-action: none;
}

.preview tosi-sizebreak {
  width: 100%;
  height: 100%;
  background: #fff8;
  border: 1px solid #aaa;
}

.preview tosi-sizebreak * {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translateX(-50%) translateY(-50%);
}

.preview .container {
  position: relative;
  min-width: 100px;
  min-height: 40px;
  max-height: 100%;
  width: 400px;
  height: 100px;
}

.preview .sizer {
  position: absolute;
  width: 24px;
  height: 24px;
  line-height: 24px;
  text-align: center;
  background: #0002;
  bottom: 0;
  right: 0;
  cursor: nwse-resize;
  opacity: 0.5;
}

.preview .sizer:hover {
  opacity: 1.0;
}

<tosi-sizebreak> supports both min-width and/or min-height, and you can of course target only one of the slots if you like. The demo site uses them to hide the bundlejs badge when space is tight.