A design with high fidelity represents important details of the intended interface, such as actual copy, typography, color, spacing and control states. It can be a static mockup or an interactive prototype. It is not proof that the final website will match every pixel in every browser.
Choose the question first
If you need to compare navigation structure, a rough sketch may be enough. If you need to check a long field label, contrast or an error state, realistic content and styling help. Detail is useful when it answers the question you are testing.
Include more than the happy path
Show empty, loading, error, disabled and success states where the interface needs them. Include long text and a narrow viewport. A polished desktop screenshot alone does not establish responsive or keyboard behavior.
Inspect the handoff
Exercise: choose a form screen and list what a developer can learn from it, then list what remains unknown. Focus order, validation rules, server failures and text alternatives often need explicit notes or a working prototype.
Do not assume every asset and line of copy has already been approved. Record those decisions and uncertainties rather than implying a project phase guarantees them. Detailed prototypes can reduce ambiguity, but can also cost time when the underlying flow is still changing.
Compare low fidelity, wireframes and usability testing.
Sources
Reference 1 provide the technical details and current compatibility information.