Insert horizontal line in html document

<hr />

'getting webbrowser content
Dim str As String = webBrowser1.DocumentText
'b is a integer to keep start index of sting that should insert into document
'PrevContent is a key string to add specified element after it 
'if you need to add hr after <span> text to add an hr after this </span>
Dim b As Integer = str.IndexOf(PrevContent) + PrevContent.Length
'insert an element in specified index
str = str.Insert(b, "<hr/>")
webBrowser1.DocumentText = str.ToString()