Formats and conversions
Tabular and structured data converts in every direction between CSV, JSON, XML, YAML and XLSX (Excel workbooks; legacy XLS and OpenDocument ODS are read too). Markdown becomes a styled HTML page, and HTML pages become clean Markdown.
Nested JSON, YAML and XML are flattened into dotted column names when the target is a table, and the main list of records is found automatically. Going the other way, CSV and Excel rows become arrays of objects keyed by the header row.
Options that matter
For CSV output you can choose comma, semicolon, tab or pipe delimiters and whether to add a UTF-8 byte-order mark so Excel shows accents correctly. For JSON you can pick 2-space, 4-space or minified output. Number and true/false detection can be switched off to keep values such as ZIP codes as text. For Excel input you can export just the first sheet or every sheet.
Private by design
Customer lists, payroll exports and API dumps are exactly the kind of files you should not upload to a random website. Here the parsing is done by SheetJS, js-yaml, fast-xml-parser, marked and Turndown running in your browser tab.