Support both src attribute and source child element

Should model support setting a model resource via either a source child element(s) or a src attribute on the model element itself?

Currently the model explainer indicates that it should be via a source child element, and specifically via a src attribute as something that was considered but not chosen.

However, I noticed that in a recent cool presentation and associated materials for a model implementation that the src attribute was the primary method indicated for setting a model. I take this as a indication that there is strong interest in supporting src for one or more reasons (it's obviously visually cleaner and easier). It would be useful to discuss what we want model to support in this regard.

Explainer:

To change the loaded resource, the author should use existing DOM APIs to add, remove or modify children elements to the element
DOM API

with the section on options previously considered

Considered Alternatives
3. A simple src="" attribute instead of children

Start with explainer.md, especially the DOM API and Considered Alternatives sections linked in the issue. Review how model resources are currently specified through source children and determine whether supporting a src attribute should be part of the design. Done means the project has a documented decision and corresponding explainer direction.