These are the full-ship shock trials, where they set off progressively closer charges to certify the hull and combat systems survive a near-miss. The tight scheduling and monitoring is largely driven by the marine-mammal mitigation requirements.

The is="" customized-built-in path (extends HTMLButtonElement) is exactly what would collapse that 500-line reimplementation. Safari refusing it is why you still fall back to a full autonomous element + ElementInternals.