Markdown is easy to start using, but it has more than one interpretation. A heading such as # Plans is familiar almost everywhere; a task checkbox or callout may work in one tool and show as plain text in another. CommonMark gives the core language a written specification and a test suite.
What CommonMark covers
The CommonMark specification defines how a shared set of block and inline patterns should be parsed. That includes headings, paragraphs, block quotes, lists, code blocks, links, images, and emphasis. Its examples are useful when a line has several plausible interpretations, such as a nested list beside a code block.
CommonMark is a parsing specification. It does not prescribe a note app's storage model, sync system, editor interface, or visual theme. A file can be plain text Markdown even if the app that opens it does not implement every CommonMark example.
Where extensions begin
Task lists, tables, footnotes, highlights, wiki links, and callouts are useful for notes. They are not all part of CommonMark's core. An app may support some of them, assign them different meaning, or leave their punctuation visible.
That is why a portable note should remain understandable when an extension is not rendered. A - [ ] task still reads like a task in plain text. A brief heading followed by paragraphs travels more predictably than a page whose meaning depends on a special layout rule.
How to check your own notes
Keep a short sample file with the syntax you use most often. Open it in your destination editor before moving an archive. Check nested lists, links to other files, image paths, tables, and tasks. For long-term access, export a few real notes and open them without the original app. The export checklist covers the rest of that test.
The live Markdown preview tool on this site uses Cindermark, a CommonMark-oriented parser with editor-focused extensions. It does not claim full CommonMark or GitHub Flavored Markdown conformance. Use the official specification when exact compatibility matters, and read Cindermark's compatibility profile for its specific limits.
For everyday writing patterns, start with the syntax guide. For how the text lives on disk, read Markdown files.