In the US, it is if you're an incorporated business serving as a "public accommodation" as defined in the Americans with Disabilities Act, which includes hotels, restaurants, theaters, storefronts. If it is just your personal blog, it doesn't apply. It is the same law that requires storefronts to have wheelchair ramps, but not homes. See Gil v. Winn-Dixie: https://scholar.google.com/scholar_case?case=674450226911160...
More than that; as someone else linked [1], if you're in a business employing at least 15 people (more than half the year), you are also required to have your site be accessible. You don't have to be serving as a public accommodation, for that.
