Start from measurable outcomes. A clause that says the solution should be accessible is too vague to test. A clause that requires conformance against the applicable accessibility requirements, identified channels, named test methods, and evidence outputs gives the buyer something enforceable and gives the supplier a clear delivery target.
For ICT heavy solutions, procurement language should identify the standards or requirement sets used, the environments tested, and the evidence that must be delivered at contract award, implementation, and acceptance. For services, it should also cover support channels, updates, and issue resolution after go live.