chore: makes types of divs more specific

[rachel-fenichel]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

Part of #5857

Proposed Changes

• Add casts when calling document.createElement('div').
• Update types for properties, return types, and parameters that can be more specific as a result.

Behavior Before Change

These were annotated as Element. The typescript compiler threw errors, saying that style and similar properties do not exist on Element.

Behavior After Change

Code works the same but tsc has fewer errors.

Reason for Changes

Improve typing/fix errors.
tsc doesn't actually care about the casts when the divs are created and used immediately, because it's clever. But tsc cares about having the correct types when the divs are saved or passed around, and the closure compiler needs the casts to agree that it's safe.

Test Coverage

Documentation

Additional Information

[cpcallen]

If only it would work to type the variable rather than the initialiser:

  const /** !HTMLDivElement */ subContainer = document.createElement('div');

Alas…