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### Some Sources of Problems for MathML

1. Broken math formulas, such as ‘$R=\{x|x$ is real $\}$’ instead of ‘$R=\{x|x \mbox{ is real } \}$’.
2. Math mode employment for presenting nonmath content, such as ‘$$\vbox{...}$$ ’ instead of ‘\begin{center}\vbox{...}\end{center}’.
3. Unmatched parentheses within entries of arrays, such as ‘\begin{array}...(... & ...)...\end{array}’ instead of ‘\begin{array}...\mathrel( ... & ...\mathrel)...\end{array}’.
4. Incorrect annotation of delimiters, such as ‘\bigl\{...\bigr|...\bigr\}’ instead of ‘\bigl\{...\bigg|...\bigr’
5. Empty bases for subscripts and superscripts, such as ‘{}^{...}, {}_{...}’ instead of ‘{\csname HCode\endcsname{}}^{...}, {\csname HCode\endcsname{}}_{...}’.
6. Use of the operators ‘^’ and ‘_’, instead of ‘\sb’ and ‘\sp’, outside the presence of TeX4ht. (TeX4ht becomes active only at the ‘\begin{document}’ command. The ‘early_’ and ‘early^’ options extend this awareness to the preambles of the source latex documents.)