Warning: Implicit string type conversion from "AnsiString" to "WideString"
DOMString is defined as DOMString in FPC 3.0.4.You are in so far right that there won't be a warning Implicit string type conversion from "AnsiString" to "WideString". However there'll now be a warning Warning: Implicit string type conversion from "AnsiString" to "UnicodeString". This is because an assignment does result in a conversion from either ANSI or UTF8 of the AnsiString to UTF16 of the UnicodeString (or WideString in the past). This might not be what the user desires (as it might result in a performance impact, especially if the codepage of the string is not UTF8 as then the conversion will likely be ANSI -> UTF8 -> UTF16), thus this warning exists.
function CreateTextNode(const data: DOMString): TDOMText;
And DOMString is XMLString which is WideString.
So the whole DOM and fcl-xml is still WideString oriented.
B.T.W. On january 20th 2019 the conversion from WideString -> UnicodeString is introduced in fcp-xml.
So in trunk and newer FPC version you won't get the warnings.