On a CPC monitor, the HSYNC is rendered in "absolute black". It is darker than the black output by the Gate-Array.
The VSYNC is also modified before being sent to the monitor. It happens two lines * after the VSYNC from the CRTC and stay two lines (same cut rule if VSYNC is lower than 4). PAL (50Hz) does need two lines VSYNC_width, and 4us HSYNC_width.
Using CRTC1, VSYNC width value 0 means a value of 16.