Support <template> on Safari <7 and Edge for new options page

[greiner]

Background

We're using <template> elements in various places in the new options page. Safari <7 and Edge doesn't support those, however. So we need to work around that limitation.

What to change

• Hide <template> tags by default
• Implement HTMLUnknownElement.content using DocumentFragment to polyfill HTMLTemplateElement.content

[oleksandr]

<template> on Edge seems to be supported: ​https://dev.windows.com/en-us/microsoft-edge/platform/status/templateelement

[greiner]

Replying to oleksandr:

<template> on Edge seems to be supported: ​https://dev.windows.com/en-us/microsoft-edge/platform/status/templateelement

That'd explain why window.HTMLTemplateElement is not defined in Edge 20.10240 (the one I tested) but not why <template>.content is also undefined. Any idea why that might be the case?

[sebastian]

After all, it shouldn't matter much, as we need that polyfill for Safari anyway. And instead for a particular browser and it's version, we should rather check for presence of HTMLTemplateElement.content anyway.

[oleksandr]

Replying to greiner:

That'd explain why window.HTMLTemplateElement is not defined in Edge 20.10240 (the one I tested) but not why <template>.content is also undefined. Any idea why that might be the case?

Both window.HTMLTemplateElement and document.createElement('template').content are defined starting from EdgeHTML 13 (November 2015). I've just checked, but it is also stated ​here

Note: referencing the Edge app version is ​advised against, since it is the underlying engine (EdgeHTML) that's important here.

[sebastian]

With the traditional Adblock Plus Safari extension being phased out in favor of a native app, and Microsoft Edge seeming to support <template> natively meanwhile, this became unnecessary.