The Open XML SDK provides tools for working with Office Word, Excel, and PowerPoint documents. This fork supports current .net versions.
$ dotnet add package Codeuctivity.OpenXmlPowerToolsvar sourceDocxFileContent = File.ReadAllBytes("./source.docx");
using var memoryStream = new MemoryStream();
await memoryStream.WriteAsync(sourceDocxFileContent, 0, sourceDocxFileContent.Length);
using var wordProcessingDocument = WordprocessingDocument.Open(memoryStream, true);
var settings = new WmlToHtmlConverterSettings("htmlPageTitle");
var html = WmlToHtmlConverter.ConvertToHtml(wordProcessingDocument, settings);
var htmlString = html.ToString(SaveOptions.DisableFormatting);
File.WriteAllText("./target.html", htmlString, Encoding.UTF8);