The most important guideline to give is the following: Write clean unit tests if there is actual value in testing a complex piece of logic in isolation to prevent it from breaking in the future Otherwise, try to write your specs as close to the user’s flow as possible

Are you also tired and fed up with the bulkiness of jQuery, but also don't want to have to type document.querySelector("div").appendChild(document.createTextNode("hello")); just to add some text to an element?

We all know that real business logic does not belong in the presentation layer, but what about simple presentation-oriented things like coloring alternate rows in table or marking the selected option in a <select> dropdown? It seems equally wrong to ask the controller/business logic code to compute these down to simple booleans in order to reduce the logic in the presentation template. This route just lead to polluting the business layer code with presentation-oriented logic.

The fears of breaking one of Svelte's core tenets seem overblown to me. Style encapsulation by default is great, but that doesn't mean we should contort the framework around it.

warn iff not seen before can be an acceptable option.

So what Python currently does in issue depreciation warnings in the main program, but not in libraries and similar code. That may also be a reasonable way to limit the number of warnings while making sure deprecations don't go unnoticed (because that makes them useless).