Open XML Formats and file name extensions - Microsoft Support
Microsoft Office uses the XML-based file formats, such as .docx, .xlsx, and .pptx. These formats and file name extensions apply to Microsoft Word, Microsoft Excel, and Microsoft PowerPoint.
Applies To:
Excel for Microsoft 365, Word for Microsoft 365, PowerPoint for Microsoft 365, Excel 2021, Word 2021, PowerPoint 2021, Office 2021, Excel 2019, Word 2019, PowerPoint 2019, Office 2019, Excel 2016, Word 2016, PowerPoint 2016, Office 2016
Description of an update for Microsoft XML Core Services 4.0 Service ...
When you visit a Web site or run an application that loads XHTML documents by using Microsoft XML Core Services (MSXML), MSXML will send requests to the World Wide Web Consortium (W3C) to fetch well-known Document Type Definition (DTD) files every time. This behavior may bring lots of traffic to the W3C server.