Why structure fails silently
A parser regression, an unusual source layout, or an encoding problem can produce Markdown that still reads well while its structure is broken. A flattened heading hierarchy leaves the prose intact; a missing separator row still renders as text in some viewers. None of these defects raise an error. They quietly degrade chunking, retrieval, and citation quality downstream, which is why an active pre-publish check is cheaper than a later investigation.
The five-point checklist
1) Heading hierarchy: exactly one H1, H2/H3 levels descending without gaps, and no footer or caption promoted into a heading. 2) Table integrity: consistent column counts, header and separator rows present, and multi-line cells not breaking the grid. 3) Links and images: targets intact and alt text preserved. 4) Code blocks: fences closed in pairs and language labels correct. 5) Front matter: if the export carries a YAML block, the --- markers are paired and the fields parse.
Where getmarkitdown.com fits in the loop
MarkItDown Online batch preview shows the heading outline of every file before you download the ZIP, and the validation gate flags files with broken heading trees or missing content, so the first two checklist items become automatic steps. Typical conversions finish in 2-5 seconds per file, and batch conversion plus sampling reviews the whole folder faster and more consistently than reading every output by hand. Feedback: sapsap@qq.com.