How to link to specific line of text in page, using only URL?

Depends on the situation:

1-Your own content

a. Simply add a href id, and then send a link to that (http://exmaple.com/document.php#myLine)

b. For a more advanced solution, you can scroll to a specific area and highlight it using javascript, basically you call something like this: http://example.com/document.php?scrollX=100&scrollY=230

and in your PHP code you have a function like this:

<?php
if(isset($_GET['scrollX']) && isset($_GET['scrollY')){
    ?>
       <script type="text/javascript">
           window.scrollTo(<?= $_GET['scrollX'];?>, <?=$_GET['scrollY'];?>);
       </script>
    <?
}
?>

You can also use JQuery and do more fancy things like highlighting text or changing CSS values depending on the arguments

2-Someone else's content

You cannot directly link to a specific line or area without a browser extension or an external service like CiteBite, however with some hackery you can build your own little service that displays the website and run your own JS on it (think iFrames or something like that)

You must give an ID to your element. Example: <div id="footer">Footer</div>. If you go to mypage.html#footer, the browser will get you there.

If it's not your webpage, look in the code, you may find an ID element next to the text you want.

As far as my knowledge goes, you cannot do it natively without some sort of caching, plugins or addons or having full control over html where you can embed anchor tags.