The \special{t4ht@[}...\special{t4ht@]} gobble the enclosed stuff. The external pair is provided as grouping mechanism for sub/super-scripts cases like \bigl(...\bigr)^x within dvimath mode. The {\HCode{}} is neded for creating content delimiters . delimiters like in $\bigl. a_b \bigr)$; without that mathml gets something wrong there. \bigl{.} et al produce empty para,etr , hence the \:EMPTY is a ompensation for such cases.