Try loading the html file into a RichEdit 
The following example shows how to search and replace in the RichEdit

Var 
  TheFile : String; 
  SearchStringPos : Integer; 
  YourSearchString : ShortString; 
  YourReplaceString : ShortString; 
begin 
  RichEdit1.PlainText:=true; 
  RichEdit1.Lines.LoadFromFile(‘YourFile.html’); 
  YourSearchString := ‘<BR>’; 
  YourReplaceString := ‘<HR>’; 
  TheFile:=RichEdit1.Text; 
  SearchStringPos:=Pos(YourSearchString,TheFile); 
  While SearchStringPos > 0 do 
  begin 
    RichEdit1.SelStart:=SearchStringPos-1; 
    RichEdit1.SelLength:=length(YourSearchString); 
    RichEdit1.SelText:=YourReplaceString; 
    TheFile:=RichEdit1.Text; 
    SearchStringPos:=Pos(YourSearchString,TheFile) 
  end; 
  RichEdit1.Lines.SaveToFile(‘YourFile.html’); 
end;