Start with a comparison someone can challenge
Two screenshots can make a persuasive sales pitch without answering a useful question. Was the old page allowed to finish loading? Are both images the same width? Is one showing the desktop layout and the other a phone? A comparison deserves scrutiny before it deserves applause.
Our proposed evidence standard starts with the original live site, captured before replacement. Keep the full-resolution files, page addresses, capture times, browser settings and viewport dimensions. Preserve a full-page image behind any smaller presentation crop. A visitor should be able to inspect what was left outside the frame.
Separate appearance from capability
A cleaner service page can demonstrate stronger hierarchy. It cannot demonstrate that an inquiry reaches the right person. A calculator screenshot shows a tool exists; a tested calculation and its stated assumptions show something about whether the tool is useful. These are different claims, and each deserves its own evidence.
For a hypothetical contractor rebuild, the capability table might record a new cost-estimation worksheet alongside a contact journey. The worksheet can be tested locally. Delivery of an inquiry needs an integration test. Neither result establishes that revenue increased. Until comparable outcome data exists, commercial improvement remains a hypothesis to measure.
Put numbers in their proper setting
Google's performance guidance distinguishes controlled lab tests from measurements of actual visitors. That distinction matters in a portfolio: a staging score under one device profile is not a report of every customer's experience. Record the test configuration, retain repeated runs and describe the scope of the comparison.
Accessibility needs similar care. W3C's evaluation guidance describes a combination of tools and human evaluation. An automated score is useful evidence, but it does not replace trying a keyboard journey or involving people with disabilities. Publish what was tested, not an unsupported claim that the entire experience is perfect.
Credit what was already good
The old site may contain excellent photography, animation, explanations or testimonials. Preserve their provenance. Reorganizing a client's existing work is valuable design work; claiming to have created it would tell a different and inaccurate story.
The strongest case study lets a prospect inspect three things separately: what changed visually, what became possible operationally and what has actually been measured. It can acknowledge an unchanged strength or an unresolved limitation. That makes the work easier to judge and the next commercial decision easier to make.