I just noticed all the code in nsHTMLStyleSheet that was added for bug 41262. This code does a bunch of things less efficiently than it should be: * it looks to me like it prevents any table style data from being cached in the rule tree, even when the rules attribute is not present * it does stuff in the post resolve callback that really should be done earlier so that it doesn't mess up the cascade I suspect that what should be done here is: * rather than mapping the "rules" attribute into style on the table's style context, do a GetAttribute on the table in RulesMatching and add the appropriate rule(s) only if needed. * rather than using a single rule object, create one rule object for each table element that has a "rules" attribute (or whatever attributes are currently being mapped into tableData->mRules, and either cache this straight on the table element or use a hashtable. This allows us to get the benefits of the rule tree even when the rules are needed. * Do the work normal MapRuleInfoInto phase rather than the post resolve callback.

Note that the rules also should be immutable to satisfy the contract of the nsIStyleRule interface (bug 188803).

the code is question got removed in bug 43178