Re: [w3c/webcomponents] Generic programs can't reliably use/manipulate documents via the DOM (#640)

> Why is it very important for each *component* to do this? Sometimes it will be what the document author intends and wants; other times not. Can we not find a compromise where the document author gets to decide which scripts can and can't access which shadow DOMs? Then, those authors have a clear idea of what might break if/when a component is updated, and what will not.

Because otherwise pages would start depending on the implementation details of components. It's possible, for example, for a graphing component to be implemented using SVG in one iteration and canvas in another. If page's code ever depended on the component using SVG, then the page would break when the component's version gets updated and it starts using canvas instead of SVG.

Ultimately, page authors have the power not to use a component which doesn't expose its shadow tree, or demand that they provide some API to mess with component's shadow tree. However, it's very important for the authors of each component to understand that doing so results in its shadow tree's structure being dependent by its users.

If the authors of a given component doesn't care about this, or is committed to maintain the backwards compatibility, then they can either not use shadow tree at all, or use an open mode shadow tree.

Either case, we don't believe we should give the unilateral power to users of components to decide whether it's okay to intrude into components' shadow tree or not.

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