MDEditor is a Markdown tool station: an editor, a viewer, converters to HTML, PDF and Word, and a table generator. Every one of them runs in the browser tab you already have open.
The design constraint is simple. Documents often contain things you would not paste into a random web service — internal specs, unreleased plans, customer notes. So none of these tools send your text anywhere. Parsing, highlighting and file generation all happen on your device, and once a page has loaded it keeps working without a network connection.
There are no accounts, no usage limits and no paid tier. The site is supported by unobtrusive advertising, which is why there is a privacy policy rather than a sign-up form.
The editor is built on the open-source md-editor-v3 component, and rendering follows CommonMark with GitHub Flavored Markdown extensions. If you find a case where the output looks wrong, that is a bug worth reporting.