Connecting a spectrum analyzer without DC block
in your example, the input power level is 10dBm max. A DC signal could be considered to be an RF signal at 0Hz. 10dBm into 50 Ohms is 0.707V RMS, so 1 VDC is above the +10dBm maximum. 1 VDC is +13dBm into 50 Ohms.

This huge signal at 0Hz can cause measurement accuracy problems with signals at other frequencies, for example compression due to front-end overload. To have DC and still have measurement accuracy, 125mV is probably the absolute maximum that I would ever put on the front end, and I would stay under 70.7mV to make a good measurement. But you should just use a DC block unless there is a good reason not to. The rating is 0 VDC for a good reason, so don't put DC on it. This is not generic advice.

I don't speak for Keysight Technologies.