Discard the stems, seeds and veins of the chiles and soak them in hot water for 15 minutes. Discard the cooking liquid and set the chiles aside.
In a medium saucepan, cook the tomatillos and garlic clove with enough water to barely cover until the color changes—about 8 minutes (do not let the tomatillos burst, though). Do not drain. Set aside to cool.
Using a slotted spoon, transfer the tomatillos and garlic to a blender along with the chiles and process to a smooth salsa. If needed, add some of the liquid from the saucepan to achieve the desired texture. Season with salt.