Learn Spanish (Latin America): Me alegro de que la miga haya quedado perfecta. - I am glad that the crumb has turned out perfect.

In this context, quedar means "to turn out" or "to end up." It emphasizes the final result of a process, like baking a loaf of bread. Using ser or estar wouldn't capture that specific sense of a finished result turning out well.

Why do we use the present perfect subjunctive (haya quedado) instead of the regular present subjunctive?

The phrase Me alegro de que triggers the subjunctive because it expresses an emotion. We use the present perfect subjunctive (haya quedado) because the baking is already a completed action, but the result has a direct connection to the speaker's present feeling of joy.

Is the de in Me alegro de que strictly necessary?

Yes. The full verb is alegrarse de (to be glad about). Dropping the de before que is a common mistake even among native speakers (a phenomenon known as queísmo), but in strict standard Spanish, the de must be kept.

Does miga just mean any little breadcrumb on the table?