Is it possible to develop SharePoint custom web parts in Office 365 Online without utilizing Visual Studio?

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Yes, it is possible to develop SharePoint custom web parts in Office 365 online without utilizing Visual Studio. There are several methods for developing custom web parts in Office 365, including using SharePoint Framework (SPFx), SharePoint Designer, and third-party tools like Nintex Forms and Flow. SharePoint Framework is a popular framework for developing custom web parts in Office 365, and it supports web technologies like React, Angular, and Vue.js. SharePoint Designer is another tool for developing custom web parts, but it has limited functionality compared to SharePoint Framework. Ultimately, the choice of tool depends on the project requirements and developer's expertise.