The standard is three things at once
"A good state of repair" is about condition — things work, and things that break get fixed.
"Fit for habitation" is about the unit being livable: heat, water, a working stove, a unit free of pests and mould, doors and windows that close and lock.
"Complying with health, safety, housing and maintenance standards" pulls in municipal law. In Toronto that is the property standards bylaw and, for buildings of three or more storeys and ten or more units, the RentSafeTO apartment building registration and evaluation programme. A unit can satisfy a landlord's own idea of good repair and still fail a standard.
It survives what the tenant knew and what the lease says
Section 20(2): "Subsection (1) applies even if the tenant was aware of a state of non-repair or a contravention of a standard before entering into the tenancy agreement." Renting a unit "as is" at a discount does not transfer the repair obligation.
A lease clause making the tenant responsible for repairs the Act assigns to the landlord does not hold. The tenant's own obligations are narrower and separate: ordinary cleanliness under section 33, and repair of damage they or their guests caused, wilfully or negligently, under section 34.
The line between landlord repair and tenant damage
Wear is the landlord's. Damage is the tenant's. A tap washer that has failed after six years is wear; a cracked sink someone dropped a weight into is damage.
That line is decided on evidence, and the evidence is almost always photographs and a dated record of what was reported and when. Deciding the line correctly and being able to prove it are different problems, and the second one is the one that loses.
Ontario has no damage deposits. A landlord may collect a rent deposit of at most one rent period, applied to the last period of the tenancy — not to repairs. So the recovery route for tenant damage is an application to the Board, which is a paperwork exercise before it is anything else.
What a tenant can do if you do not repair
A tenant can apply to the Landlord and Tenant Board under section 29. Section 30 lets the Board order the repair, order the landlord to comply with standards, order a rent abatement, authorise the tenant to do the work and deduct the cost, or prohibit a rent increase until the work is done.
A tenant can also call municipal standards enforcement, which is often faster than the Board and produces a work order with its own deadline.
The defensible position is not a perfect building. It is a documented one: what was reported, when you learned of it, what you did, and when.