Photonic networks based on wavelength-selective switches (WSSs) can transport wavelength-division-multiplexed (WDM) signals in a cost-effective manner. To accommodate the ever-increasing network traffic, the spectral efficiency should be maximized by minimizing the bandwidth of the guardbands inserted between WDM signals. Quasi-Nyquist WDM systems are seen as offering the highest spectral efficiency in a feasible way. However, highly dense WDM systems suffer from the signal-spectrum narrowing induced by the non-rectangular passbands of WSSs. Furthermore, widely deployed WSSs cannot process quasi-Nyquist WDM signals since the signal-alignment granularity does not match the passband resolution of the WSSs. This paper proposes a network architecture that enables quasi-Nyquist WDM networking with widely deployed WSSs. Through intensive network analyses based on computer simulations, we confirm that it has 30.8% higher spectral efficiency than conventional networks. Its feasibility is verified by transmission experiments on 72-channel 32-Gbaud/400-Gbps dual-carrier dual-polarization 16-ary quadrature-amplitude-modulation signals aligned with 66.6-GHz spacing in the full C-band. The net fiber capacity of 28.8 Tbps, the transmission distance of 900 km, and the hop count of 9 are attained by our proposed quasi-Nyquist WDM networking scheme.