Here are the Highland Park zoning ordinances. No one is going to haul you away in handcuffs for renting a room from a homeowner.
These ordinances about non-related people cohabiting or renting an outbuilding are basically to prevent people from turning their homes into transient hotels, flophouses, frat houses, day labor camps, etc.
No. They are very serious about it. I am not saying anyone is going to be taken away in handcuffs, but ther rules are enforced. Many homeowners would rather keep good relations with the "town" rather than test the intent of the laws. They do enforce the cohabitation (at least in University Park and I can only expect in Highland Park) with exorbident fines and penalties. I have personal experience with this.
Keep in mind that Highland Park and University Park are autonomous cities within the city of Dallas. They maintain their own courts, police, fire, and school district. Living there is more like a small town than living in a big city.