Awesome

attachshadow

An iframe based Shadow DOM poorlyfill

Features

• compatible down to IE9 with a size of about 1.3K
• provides an element.attachShadow(mode) method out of the box
• both mode open and closed are available. open is set to the shadowRoot property as expected
• events are synthetically re-targeted to the owner, hence propagated to the parent document
• encapsulation is preserved, including :host CSS declarations
• each iframe transparently resizes accordingly to its content

Missing

• the <slot> element <small>(under consideration)</small>
• if you shadowRoot.innerHTML = "<p onclick='console.log(0)'></p>" DOM Level 0 events won't ever propagate (inconsistent with real Shadow DOM, just don't do that or simply use hyperHTML instead that will use addEventListener)

hyperHTML(el.shadowRoot)
  `<p onclick=${() => console.log('🎉')}></p>`;

What this is not

• this is not a Custom Elements or an HTMLTemplateElement polyfill. This is only the attachShadow method.
• this is not, and never could be, a 100% fully standard Shadow DOM polyfill, for the simple reason that Shadow DOM cannot be polyfilled (this is a poorlyfill indeed, with its caveats and compromises)
• not yet a battle tested solution. If you have any experience with iframe shenanigans please help making this poorlyfill less poor, thank you!

CDN

You can use https://unpkg.com/attachshadow as CDN for this library.

Test

Please visit the test page and click on the button to see in console the propagated event in IE9 or any other browser.

You can also test more modern browsers in this good old CSS Tricks demo.