https://www.marylandtaxes.gov/forms/Personal_Tax_Tips/tip56.pdf
https://cpadelaware.net/f/if-i-work-in-pa-nj-or-md-but-live-in-de-do-i-pay-taxes-in-both
For Delaware, you calculate your return as both a non-resident and a resident, and file whichever form gets you a better answer.
https://revenuefiles.delaware.gov/2023/PIT-RES_TY23_2023-01_Instructions.pdf
https://revenuefiles.delaware.gov/2023/PIT-NON_TY23_2023-01_Instructions.pdf
For Maryland, you file both a resident and a non-resident return.
https://www.marylandtaxes.gov/forms/23_forms/Nonresident-Booklet.pdf
https://www.marylandtaxes.gov/forms/23_forms/Resident-Booklet.pdf (MD is sure this is the correct link, but it isn't working for me. I don't have any other official source of the forms instructions, though.)
