When sharing a window, only the rasterized pixel output of that window is captured.
Therefore, if you need a high resolution version of your web page, the window that is opened must also be sized appropriately.
Additionally, video is particularly tricky because capturing video from a window requires a great deal of CPU and isn't normally done at full frame rate. Codecs used with screen sharing are optimized for a low frame rate, higher resolution, and quality. Because of these limitations, you are basically never going to get a good quality video shared from a web page to stream correctly over Zoom or any similar software.
You'll have to either make-do with the quality that exists now, or you'll have to skip the screen-sharing route entirely.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…