united-states
Is it the responsibility of the board of an HOA to enforce county and
state laws about blocking a sidewalk.
This could go either way, given the limited information available in the question.
A Homeowners Association (HOA) has no responsibility to enforce county or state laws. And, an HOA can't directly enforce state laws and local ordinances. HOA officials, either in their official capacity, or merely as private citizens, can always call the appropriate state or local officials to enforce a state or local law, if a violation of that law is observed, to inform the government officials of this violation. And often, HOA officials do just that.
But an HOA may, and sometimes an HOA has a duty to, enforce the HOA's Covenants, Conditions, and Regulations (CCRs), and any Rules and Regulations that the Board adopts with the authority granted to it by the HOA's governing documents or by state law.
At most, it can impose civil fines pursuant to its governing documents on its members for the reasons stated in the governing documents according to the process set forth in the governing documents and the relevant provisions of state law. If these fines aren't paid they can usually be enforced in a civil lawsuit (often in small claims court or a limited jurisdiction court), or by making it a lien against the property which it may foreclose upon someday. Also, members with fines that have not been paid in a timely manner can often be barred from using certain common amenities of the HOA that aren't absolutely necessary for the use of a member's property in the HOA, and can often be barred from voting in HOA elections.
But, the HOA could reasonably interpret the HOA's CCRs in a manner that treated "parking in a driveway" which it requires, to mean "parking in a space that excludes the sidewalk", which would typically be punishable by an HOA fine if violated and upheld following an HOA hearing if the person fined made a timely request for a hearing. A court would normally defer to any reasonable interpretation that an HOA Board makes of its own CCRs, although the standard of review of Board interpretations of CCRs varies from jurisdiction to jurisdiction.
Also, most HOA CCRs contain a general clause that require members of the Homeowners Association to use there property within the HOA in a manner consistent will all applicable federal, state, and local laws. If parking in a way that blocks a sidewalk violates a state law or a local ordinance, then this would also be a violation of the HOA's CCRs.
Finally, most HOAs have governing documents that authorize the Board of the HOA to adopt reasonable rules and regulations regarding the use of common elements and limited common elements in the HOA. A sidewalk, even when it crosses a driveway, would usually be a common element or a limited common element of the HOA that would be subject to the reasonable rules and regulation adopted by the Board. So, if the Board adopted a rule or regulation prohibiting parking in sidewalks (which, since it is illegal anyway, would almost certainly be legal for it to adopt), the HOA could enforce this rule or regulation with civil fines and its other enforcement rights, even if this rule was not found in the recorded CCRs of the HOA.
Beyond the confines of rules that apply only in HOAs, in theory, any pedestrian who regularly uses a sidewalk that is often blocked by a car in the driveway of that car owner's home could also sue the person whose car is blocking the driveway for interfering with an easement (i.e. a legal right to cross or otherwise make limited use of part of someone else's property) that they are entitled to use, either seeking money damages or injunctive relief (i.e. a court action directing someone to take or refrain from particular actions which is ultimately punishable by contempt of court if the court order is violated). Sidewalks would almost always be associated with some form of express or implied easement across the property of the driveway owner.
