And small tag is obsolete, and throws another linter error Help:Extension:Linter/obsolete-tag
Topic on Help talk:Lint errors/html5-misnesting
small is not an obsolete tag. It is not listed on the help page you linked there.
I am sorry, i didn't noticed. Although i am completely sure "big" and "small" were considered obsolete before, as i personally adapted templates on my wiki for compliance with this.