Every digital project will eventually be handed to someone who was not involved in building it. The quality of that handoff — and the quality of the documentation and conventions that make it possible — determines whether the work continues to serve its purpose or becomes a liability.
What maintainability actually means
Maintainability is about whether the next person who touches the work can understand it, extend it, and fix it without breaking things. A maintainable website has a clear content model. A maintainable codebase has consistent conventions. A maintainable design system has documentation explaining not just what the components are but why they were built the way they were.
Documentation as a deliverable
Documentation is frequently treated as an optional extra. This is backwards. Documentation is a deliverable, and it should be scoped and budgeted as one. At minimum: a content guide, a technical readme, and a design guide.
The conventions that matter most
The conventions that matter most are the ones that affect the most common tasks. Document them. Not because every developer will follow them perfectly, but because having them written down makes it possible to have a conversation about deviations.