What is an abandoned document?

Abandoned documents are effectively leaked: they are Document objects which have never been destroyed, and persist after loading about:blank, running a garbage collection and clearing caches. This is probably because there's some object that is holding a reference to the Document object, possibly in the GC heap, or via a retain cycle (often involving Nodes in the document).

All documents are referenced by Document::allDocumentsMap(), so they are not leaked in the sense that the 'leaks' tool doesn't show them.

Why are layout test results showing me a list of abandoned documents?

Because leaked documents usually entrain a lot of other objects, which can use lots of memory (e.g. via entries in the memory cache). Leaking documents is bad because it will cause ever-increasing memory use as the user browses. See ​https://bugs.webkit.org/show_bug.cgi?id=186214

What should I do if see a new case of document abandonment?

If you made a code change that is causing a test to newly show that a document is leaked, it probably means you have a coding bug that is triggered a leak or (more likely) a reference cycle. You need to resolve this before committing.