Downward facing dog (DFD) is probably the most commonly cued pose and often the “resting” pose.
Heels are supposed to touch the floor
Not exactly! A student with tight hamstrings or short Achilles tendon* may find heel-to-floor contact challenging. Hamstrings are attached to the sit bones and if they are tight, pelvis will be pulled into a tucked position which strains the lower back. If you have tight hamstrings, save your hamstring stretching for poses that will actually change the length of the muscles instead of pulling other body parts out of alignment and causing unnecessary strain. A student with less range of motion in his/her ankles (reduced dorsiflexion) may not be able to have heel-to-floor contact.
*Achilles tendon is the largest and strongest tendon in the body. When the calf muscles flex, the Achilles tendon pulls on the heel. This movement allows us to stand on our toes when walking, running, or jumping. Despite its strength, the Achilles tendon is also vulnerable to injury, due to its limited blood supply and the high tensions placed on it (https://www.webmd.com/fitness-exercise/picture-of-the-achilles-tendon#1)
Legs have to be straightened
There seems to be an understanding that the “correct” form” of DFD must be done with straight legs. Student tend to straighten their legs when comparing to others. This may cause rounding of backs and taxing the hamstrings. A bent-knee down dog is as much a down dog as a straight-legged down dog.
In conclusion, don’t worry about trying to get the heel-to-floor contact. The “correct” form should be the one that best serves you and at any moment. If your hamstrings are tight, it is ok to bend your knees as much as required as long as you maintain a long spine with pelvis tilted up towards the ceiling.
Go forth and have a more sustainable DFD 🙂