By default, Mammoth maps some common .docx styles to HTML elements. For instance, a paragraph with the style name Heading 1 is converted to a h1 element. If you have a document with your own custom styles, you can use an embedded style map to tell Mammoth how those styles should be mapped. For instance, you could convert paragraphs with the style named WarningHeading to h1 elements with class="warning" with the style mapping: