All B_Hbond length in diborate are
equally due to resonance.
Answers
Resonance i.e. delocalization of electron charge cloud in conjugated systems ( may be long conjugation or just one ) — Resonance is a very prominent permanent effect; if conjugation involved with phenyl/ benzene ring , then that's known as Mesomeric Effect. Conjugated double bonds ( like allylic system, benzene ring system etc) take part in Resonance ( not cummilated double bond like allenes , or isolated double bonds like in case of ethene etc, but 1,3-butadiene can take part in Resonance ).
In Resonance as π-electron cloud or non-bonding electrons (sometimes single electron takes part in resonance, like allylic radical) are involved in conjunction with vacant-orbital of the corresponding element/species, bond order of involved double bond decreases and b.o. of involved single bond increases . You can easily see it if u consider their MO diagrams. So, in respect of bond length, bond length of single bond decreases as it gets some double bond character and bond length of double bond increases as it losses it's some double bond character ( you can use partial double bond term here). For illustration I just give you two examples,