It is a legal and moral obligation to make web content and functionality available to people with disabilities.
Primarily, write semantic HTML. Without this rock solid base for your content, any later efforts to bolt on accessibility will fall short. Developers must familiarise themselves with the HTML living standard so that they use the best element for each piece of structure, content, and interaction.
All Barnardo's web pages must achieve WCAG (web content accessibility guidelines) AA conformance throughout, and level AAA conformance where possible. Restated: AA conformance is a minimum, not a goal.