While the standard Prolog flags are identified in Environment Control (Prolog flags), external code such as libraries and packages can also make use of Prolog flags.
In reading load_html/3 noticed
Calls load_structure/3 with the given Options, using the default options
dialect(HTMLDialect), where HTMLDialect is
html5(default), depending on the Prolog flag
Checking the value after the library is loaded
?- current_prolog_flag(html_dialect,Dialect). Dialect = html5.
Checking the value before the library is loaded
?- current_prolog_flag(html_dialect,Dialect). false.