A visitor fills in a form and finds a grey button that does nothing. The page may be waiting for a required field, a file upload or a checkbox, but the visitor cannot see which one. Disabling the action has prevented submission without explaining the next step.
W3C’s form-instructions tutorial recommends clear guidance about the information a form needs. Requirements should be understandable before someone reaches the end, not discovered by trial and error.
Explain the condition near the task
If a file is required, say so beside the upload field and identify the accepted formats. If a date must follow another date, explain that relationship in ordinary language. Do not rely only on a colour change or a visual symbol with no clear meaning.
Consider whether allowing an attempted submission with useful validation would be more understandable than disabling the button. That choice needs testing with keyboard and assistive-technology users as well as mouse users; a control that cannot receive focus may make an attached explanation hard to discover.
Test the incomplete states
Leave one requirement unfinished at a time. Can you tell what is missing, reach the relevant field and correct it without losing the information already entered? Repeat the test on a narrow screen, where the message and button may be far apart.
Distinguish incomplete input from an action already in progress. A temporary sending state needs its own feedback and a clear result. It should not leave the visitor wondering whether another click will create a duplicate request.
Use specific wording such as “Choose a delivery date to continue” when that is the actual condition. Avoid a generic “Invalid form” message that gives no route forward. The aim is not to decorate an unavailable button; it is to help a person understand and complete the task that makes the action possible.