To address these challenges, a new quantum theory, Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory (MRSF-TDDFT; MRSF for brevity), has been developed. MRSF is a versatile platform that incorporates both the static multi-configurational effect and dynamical electron correlation into the description of the ground and excited electronic states within the computationally efficient linear response theory. In a series of studies, it has been demonstrated that the MRSF approach can yield accurate nonadiabatic coupling matrix elements (NACMEs), enabling reliable nonadiabatic molecular dynamics (NAMD) simulations. Additionally, MRSF provides a topologically correct description of conical intersections, accurate values of singlet-triplet gaps, accurate spin-orbit couplings (SOC), and very accurate X-ray absorption predictions. This method has also been successfully used in designing high-performance optoelectronic materials. A perspective further underscores the merits of MRSF in this context. Recent efforts to optimize exchange-correlation (XC) functionals specifically designed for MRSF can further improve its performance. The essence of MRSF lies in the utilization of responses from two components of the triplet state, as opposed to the single-component formulation of SF-TDDFT. This doubles the response space without introducing multi-reference orbital optimization found in Multiconfiguration Pair-Density Functional Theory (MC-PDFT), or requiring expensive high-rank excitations from a single reference, as in Spin-Adopted (SA) SF-DFT. Due to its formulation, MRSF recovers important double excitations, addressing the primary issue of missing doubly excited states in TDDFT without incurring the high computational cost associated with full doubles. As a result, MRSF maintains the efficiency of linear response theory with simple single-reference orbital optimization while leveraging the advantages of multi-reference theories, making it both practical and potentially accurate.