\def\@mathbb#1{%
\ifx#1\@empty\let\@next\@empty\else\@doublestruck#1\let\@next\@mathbb\fi
\@next}
\newcommand{\mathbb}[1]{\@callprim{\@mathbb}{#1\@print{\@empty}}}
\DeclareSymbolHtml{\ulcorner}{X231C}
\DeclareSymbolHtml{\urcorner}{X231D}
\DeclareSymbolHtml{\llcorner}{X231E}
\DeclareSymbolHtml{\lrcorner}{X231F}