DOM Shim is automatically inferring serializable option as true when calling attachShadow

[thescientist13]

Type of Change

Bug

Summary

To help facilitate the contributions and enhancements around #177 and #170 , it was still an open item that users would not have to opt-in to auto-wrapping of the <template> tag when instantiating their Shadow Root in a custom element.

To illustrate, just doing this will automatically wrap the output of WCC in a <template> tag for you

this.attachShadow({ mode: 'open' });
this.shadowRoot.innerHTML = '<p>html goes here</p>';

This is based on a combination of Web APIs that we understand to work in tandem, which we are not accurately honoring ( though say some may say that honoring the spec would come at the expense of developer experience ;) )

• https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow#serializable
• https://developer.mozilla.org/en-US/docs/Web/API/Element/getHTML#serializableshadowroots

For getHTML the spec states that we can consider serializing shadow roots if serializable is set to true, e.g.

this.attachShadow({ mode: 'open', serializable: true });
this.shadowRoot.innerHTML = '<p>html goes here</p>';

I am OK with WCC automatically setting serializableShadowRoots: true

Details

I am open to being persuaded to support implied auto-wrapping, but would rather allow a compiler level option to set these automatically instead (through a new issue / PR) but for the sake of this bug report, the expectation would be that the DOM Shim only serializes if it detects serializable: true has been set in the custom element.

This might require "unbreaking" a couple test cases that were changed in #177 but that is fine. Either way we should follow the specs or otherwise document deviations from spec'd behavior, in which case if I am talked out of this, we should document on the home page docs that we do this under attachShadow in the Key Features section.