Your WebGL export is using a different aspect ratio than the one you're using in your editor.
The WebGL viewport is 960x600, an 8:5 aspect ratio, where the width is 1.6 times the height.
Your editor viewport is 650x366, so the width is 1.776 times the height.
By default, Unity scales the view to preserve the vertical axis, so you see the same height of your scene in both views. But because your editor view is wider, you're able to see more off to both the left & right sides.
So, some possible fixes you can use:
Change your export aspect ratio / target resolution to reflect your intended cropping.
In the Resolution drop-down at the top-left of your Game window in the editor, choose the resolution or aspect ratio you intend to build for, so you can design your scene with the final cropping in mind.
Add a script to your camera to adjust the vertical extents, rather than the horizontal extents, or add letterboxing/pillarboxing to match to your desired aspect ratio even when the screen/browser window is a different shape.