Principle 2 Operable · Guideline 2.5: Input Modalities
2.5.6 Concurrent Input Mechanisms
The available input method must not be restricted.
What this means in practice
Plugging a keyboard into a touch device should allow both. Sites that permit only one input mode based on device detection fail this.
Normative wording
The binding text lives at the W3C. What you read here is our own faithful rendering – not a quotation.