Keep the source identical
GitHub renders Markdown with its own conventions and site context. A desktop app may support many of them, but a badge URL, repository-relative image or GitHub alert can behave differently. Before comparing screenshots, make sure both surfaces use the same commit or the same local file. Otherwise you are testing two versions of the document.
Use the rendering corpus to isolate features without someone else’s README. Check task lists and tables first. Then try a fenced code block, Mermaid source, math and a local SVG. Record what GitHub shows, what the app shows, and whether the file path exists in both environments. GitHub’s Markdown documentation explains its own formatting and repository context.
Separate syntax from service features
A relative link may work on GitHub because the entire repository is present but fail after you downloaded only one file. An image may point to an external service, so a local offline view cannot fetch it. An app can render the content without matching GitHub’s CSS, fonts or page width.
This is a method, not a claim that Aero Markdown and GitHub produce identical output. Aero’s native renderer supports GitHub alerts and other formats, but a public release and side-by-side screenshots are still pending. For paths, use the local-image guide.