search for html text A. when not found insert html-text

You can certainly do it. Now the main challenge is where exactly in your HTML page you want to insert that "Imprint"? That's why search&replace (what stream profile does) works best. This does not require you to know the exact byte location where to insert your content. Even when you do know, any minor change in HTML layout could mess up things and you may see "Imprint" at a wrong location. If the place of insertion is always at the start or very end, that would ease things considerably. In all other scenarios, I'd refrain from using iRules, or try to find a way to go with search&replace.