rule: Expressions can occur in text content or in attribute value positions.

would it be worth while (and, tractable) to have a rule that enforces the above lit-html constraint? the following concocted code, for example, causes an exception:
````
render(html`

`, $('body')[0])
````

while the following produces happiness
````
render(html`

`, $('body')[0])
````
(if that's what you were looking for.)

cheers.