SYMPTOM:
In Strategy Web 10, when creating a new HTML container, it shows http:// by default:

Replace it with HTML tag like: <b>This is bold</b>

Run in express/editable mode, error will show:
Server Error:
404 - File or directory not found.
The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.

While in Strategy Web 9.4.1, same HTML tag is displayed fine.

CAUSE:
This is a known issue of Strategy Web 10.
ACTION:
This issue has been addressed in Strategy version 10.1. Please upgrade to Strategy version 10.1 to take advantage of the fix.