How to filter modded servers in the server browser?

No Valve games had official servers and automatic matchmaking prior to Team Fortress 2 (may have been added in updates) to my knowledge. It's possible to disable downloading custom files in multiplayer settings, but this only prevents you from connecting to a server running a custom map, not to a server running mods (the infamous "ERROR" sign is displayed if any custom textures are in use)! For instance in Day of Defeat: Source, "parachute" and "medic" mods are commonly used on active servers — essentially tweaking the gameplay based on the host's preferences. I'm not aware of a way to filter or even mark the servers in the browser, the only method I know is to always check if the server is running an official map. "Tags" can be used to inform players about mods, but I don't know if any are automatically added to a modded server.