Default HTML editor in Visual Studio 2019 doesn't include preview:
You can select the Web Forms editor right clicking on an .html file in Solution Explorer and selecting Open With...:
It includes the Design view, but requires updates as you change code and doesn't support full modern HTML capabilities:
The HTML 11 extension for Visual Studio 2017/2019 provides desktop and mobile preview tool windows automatically updated as you type:
It uses Chromium Embedded Framework v75 for HTML rendering and lets you move the preview outside the main Visual Studio window, e.g. on a second monitor.
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