Re: Important nitpick...
Nylon webbing and Velcro had been added well beyond spec by astronauts and launch pad crew. This was to hold things that flew around during ascent and zero g. Harmless with normal atmosphere or orbital pure oxygen pressure of 5-6 lbs/square inch.
However, during the fatal test, the capsule was filled with pure oxygen to normal atmospheric pressure at sea level, and then a further 5-6 lbs/square inch of oxygen to simulate relative pressure within the capsule vs. The vacuum of space. At that combined pressure of pure oxygen, all that Velcro and nylon burned fiercely.
Following the accident, I believe NASA reverted to using normal atmosphereic gases instead of oxygen. Plenty enough for the crew to breath, still worked to test retention of pressure , and no increased flammability of materials in the capsule.