Manifest and version
imsmanifest.xml present at the root, well-formed, and declaring SCORM 1.2 or 2004 with its edition.
Many SCORM imports fail because the package structure is invalid. The LMS unzips the file, reads imsmanifest.xml, and follows its references. Common faults include a nested manifest, a renamed resource identifier, a missing launch file, and a file-name case mismatch.
Those are exactly the checks this validator runs. It reads the central directory of the zip, parses the manifest, resolves every item, dependency, and file reference against the archive’s actual contents, and reports each break with the identifier involved: the difference between "import failed" and knowing which line of the manifest to fix.
Warnings cover the faults an LMS tolerates but a team should not: no resource marked as a SCO (so nothing will ever report progress or score), missing titles that render as blank menu entries, and an undeclared schema version that leaves the LMS guessing which spec to apply.
Your browser reads the package locally. The validator reads the manifest as text and the file list as a table of contents. It does not upload, execute, or render package content.