I explained that HTML, as a basic rule, marks up content with opening and closing tags to form elements.
Literal
HTML [topic-は], basic rule [as-として], opening-tag [and-と] closing-tag [with-で] marking-up element [become-とする] [quotative-と] explained.
A meta-explanation referring back to previous text — note the ~と explained ending, quoting what the author said earlier. ~として means 'as X', 'in the capacity of X'. The pattern X とする means 'treat as X', 'define as X', 'take to be X'. Note the long nominal-heavy noun phrase before the final verb, typical of explanatory writing.