According to Bug 1234403 Comment 14, the returned string of pseudoType is not correct. It should be "::after" or "::before", instead of ":after" or ":before". We have to prepend an extra ':' to the pseudoType returned from nsCSSPseudoElements::GetPseudoAtom().

Comment on attachment 8720725 [details] [diff] [review] Prepend an extra colon to the pseudo type string Review of attachment 8720725 [details] [diff] [review]: ----------------------------------------------------------------- ::: dom/animation/CSSPseudoElement.h @@ +44,5 @@ > "All pseudo-types allowed by this class should have a" > " corresponding atom"); > + aRetVal.Assign(char16_t(':')); > + aRetVal.Append( > + nsCSSPseudoElements::GetPseudoAtom(mPseudoType)->GetUTF16String()); I will add a comment for this part after the review: // Our atoms use one colon and we would like to return two colons syntax for // the returned pseudo type string, so serialize this to the non-deprecated // two colon syntax.

Comment on attachment 8720725 [details] [diff] [review] Prepend an extra colon to the pseudo type string Review of attachment 8720725 [details] [diff] [review]: ----------------------------------------------------------------- r=birtles with comments addressed. ::: dom/animation/CSSPseudoElement.h @@ +44,5 @@ > "All pseudo-types allowed by this class should have a" > " corresponding atom"); > + aRetVal.Assign(char16_t(':')); > + aRetVal.Append( > + nsCSSPseudoElements::GetPseudoAtom(mPseudoType)->GetUTF16String()); This is a bit expensive. GetUTF16String() returns a char16ptr_t.[1] Then we pass that to Append without specifying a length.[2] As a result we end up in Replace with aLength == -1 [3] so we'll call nsCharTraits<char16_t>::length(aData)[4] which will make us iterate through the whole string to get its length even though the atom already knows its length. At very least we could call: nsIAtom* pseudoAtom = nsCSSPseudoElement::GetPseudoAtom(mPseudoType); aRetVal.Append(pseudoAtom->GetUTF16String(), pseudoAtom()->GetLength()); Better still, we should do: aRetVal.Append( nsDependentAtomString(nsCSSPseudoElements::GetPseudoAtom(mPseudoType))); [1] https://dxr.mozilla.org/mozilla-central/rev/0629918a09ae87808efdda432d7852371ba37db6/xpcom/ds/nsIAtom.idl#47 [2] https://dxr.mozilla.org/mozilla-central/rev/0629918a09ae87808efdda432d7852371ba37db6/xpcom/string/nsTSubstring.h#535 [3] https://dxr.mozilla.org/mozilla-central/rev/0629918a09ae87808efdda432d7852371ba37db6/xpcom/string/nsTSubstring.cpp#535 [4] https://dxr.mozilla.org/mozilla-central/rev/0629918a09ae87808efdda432d7852371ba37db6/xpcom/string/nsCharTraits.h#292

(In reply to Brian Birtles (:birtles) from comment #4) > Better still, we should do: > > aRetVal.Append( > nsDependentAtomString(nsCSSPseudoElements::GetPseudoAtom(mPseudoType))); > Thanks for explaining the performance problem. Using nsDependentAtomString() is more concise.