You can't convert text to an image using raw HTML or CSS you will almost certainly need something like ASP or PHP, or if you're REALLY desperate then
Java(script) the latter is a seriously gross solution though.
Anyway, someones posted about ASP which I don't know much about. If you need a PHP solution here's what you should look up:
You will need GD set up on your PHP (Instructions here)
And to write text to images you will use the imagettftex function in PHP Syntax and Example code.
Here is their example code:
PHP Code:
<?php
// Set the content-type
header('Content-type: image/png');
// Create the image
$im = imagecreatetruecolor(400, 30);
// Create some colors
$white = imagecolorallocate($im, 255, 255, 255);
$grey = imagecolorallocate($im, 128, 128, 128);
$black = imagecolorallocate($im, 0, 0, 0);
imagefilledrectangle($im, 0, 0, 399, 29, $white);
// The text to draw
$text = 'Testing...';
// Replace path by your own font path
$font = 'arial.ttf';
// Add some shadow to the text
imagettftext($im, 20, 0, 11, 21, $grey, $font, $text);
// Add the text
imagettftext($im, 20, 0, 10, 20, $black, $font, $text);
// Using imagepng() results in clearer text compared with imagejpeg()
imagepng($im);
imagedestroy($im);
?>
You can modify that slightly to create a re-usable class. For example if I do this:
ImageText.php
PHP Code:
<?php
// Set the content-type
header('Content-type: image/png');
// Create the image
$im = imagecreatetruecolor(400, 30);
// Create some colors
$white = imagecolorallocate($im, 255, 255, 255);
$grey = imagecolorallocate($im, 128, 128, 128);
$black = imagecolorallocate($im, 0, 0, 0);
imagefilledrectangle($im, 0, 0, 399, 29, $white);
// The text to draw
$text = $_GET['text'];
// Replace path by your own font path
$font = 'arial.ttf';
// Add some shadow to the text
imagettftext($im, 20, 0, 11, 21, $grey, $font, $text);
// Add the text
imagettftext($im, 20, 0, 10, 20, $black, $font, $text);
// Using imagepng() results in clearer text compared with imagejpeg()
imagepng($im);
imagedestroy($im);
?>
I would be able to do this in html:
Code:
<img src="ImageText.php?text=HELLO!">
No prizes for guessing what the text says
Nifty eh?
Tweak as necessary 
Hope that helps!
Edit
Hmmm I might not fully understand the question re-reading it. The above is a solution to directly convert some text into an image that displays the said text. For hiding text in-line in the browser you should look into DHTML (Dynamic HTML) which DOES use Javascript, which is probably the only way you could do this if said text has to be written out to the browser in the first place (if you're using a preprocessor like PHP or ASP you should definitely process said text out in advance). Unlike CSS, DHTML will re-write the page once it has already been presented, thus removing the said text from the page (and replacing it with something else completely) rather than just hiding it with CSS.