1. select some text in HTML document
2 Format - Character - Font Effects tab - Effects - Hidden
a. selected characters are no longer visible (expected)
b. View->HTML source (selected characters marked in a span and display: none) (expected)
c. No way to make the hidden characters visible in document (unexpected).
There should be a "Hidden characters" option under "Display Formatting" in Tools-Options-Writer/Web-Formatting Aids, which shows the hidden characters (similar to this option for Writer - Formatting Aids).
It is true -> NEW