get page source substring when something selected on THtmlviewer

[Sniper]

Code: Pascal  [Select][+][-]

1. pstart: integer = 0;
2. plength: integer = 0;
3. page_source: UTF8String;
4. Label1.Caption := 'Code page = '+IntToStr(HtmlViewer1.DocumentCodePage; //65001 so it is UTF8;
5. page_source := HtmlViewer1.DocumentSource;
6.  
7. pstart := HtmlViewer1.SelStart; // works fine
8. plength := HtmlViewer1.SelLength; // when return negative we need to do abs(plength)
9.  
10. IntToStr(HtmlViewer1.FindSourcePos(start)); // works fine

Now, this is my problem(question): When something selected on THtmlviewer I need to get HTML code substring by mask

10 symbols + selected + 10 symbols Ex. if selected 'sometext' result text should be 'body><div>sometext</div></bo'

Made simple project (need to install Htmlviewer from package manager)

[Sniper]

Any advice, please?