It’s worth noting that this is not actually explicit in the standard so much as it is just a natural consequence of not including the half-line. Since the raster is continually moving across and down, including during all retraces (until the actual blanking pulses reset them), the presence of the half-line means that the horizontal retrace doesn’t reset at the end of the half line, it just continues scanning horizontally while the vblank pulse resets the vertical deflection to the top of the screen. Similarly, all scan lines are actually very slightly tilted down on all CRTs unless there is compensation for it. The vertical deflection is consistently moving across the length of the horizontal trace.

Edit: if you have a set that is miscalibrated or worn out, you can end up actually seeing the retrace lines during what should be blanking