Expert-led evaluation complements user testing. Heuristic evaluation is an expert review method in which evaluators assess a user interface against a set of established usability principles, known as heuristics, to identify problems. The most widely used set was developed by Jakob Nielsen in 1994 and remains the industry standard. These ten heuristics include visibility of system status, meaning the system should always keep users informed about what is happening through appropriate feedback such as loading indicators and success messages; match between system and the real world, meaning the interface should use language, concepts, and conventions familiar to users rather than internal jargon; user control and freedom; consistency and standards; error prevention; recognition rather than recall; flexibility and efficiency of use; aesthetic and minimalist design; help users recognise and recover from errors; and help and documentation. Typically, three to five evaluators working independently will find around 75 to 80% of usability problems. A related expert method, the cognitive walkthrough, has experts step through tasks from the user's perspective, asking at each step whether the user will know what to do and whether they will understand the feedback.
Several classic UX principles and laws help explain why certain designs feel intuitive. Fitts's Law states that the time to reach a target is a function of the distance to it and its size, so larger and closer targets are faster to click, implying that important buttons should be made large and accessible. Hick's Law states that the time to make a decision increases logarithmically with the number of choices, so reducing options speeds up decisions. A mental model is a user's internal representation of how something works, and designs that match users' mental models feel intuitive while mismatches cause confusion. Progressive disclosure is a design pattern that shows only essential information first and reveals more detail on demand, reducing cognitive load, which is the total amount of mental effort required to use an interface. Finally, the aesthetic-usability effect describes the observation that users perceive aesthetically pleasing designs as more usable, so good visual design creates positive first impressions and increases tolerance for minor issues.
Accessibility is a non-negotiable dimension of usability. The Web Content Accessibility Guidelines, or WCAG, are international standards published by the W3C for making web content accessible to people with disabilities. They are organised around four principles known as POUR: content must be Perceivable, Operable, Understandable, and Robust enough to work with assistive technologies. WCAG defines three conformance levels: Level A as the minimum, Level AA as the recommended standard for most sites, and Level AAA as the highest, which is generally not required for entire sites. For Level AA, normal text must meet a colour contrast ratio of at least 4.5:1, while large text (18pt or larger, or 14pt bold or larger) requires at least 3:1.
Alt text, or alternative text, describes an image for screen readers and should convey the image's purpose rather than just announcing "image of". Common accessibility issues include missing alt text on images, insufficient colour contrast, and keyboard-inaccessible interactive elements. Screen readers such as JAWS, NVDA, and VoiceOver are assistive technologies that read aloud the content of a screen for people with visual impairments, and ensuring compatibility with them is central to creating inclusive experiences for all users.