If you mean "without inserting any extra html" in your server side HTML, then no.
But, you can achieve this by wrapping the keyword with jQuery.
Take a look to the jQuery Highligth plugin which will look for and wrap into a span your keywords, you'll just need a bunch of CSS.
You will need to insert some markup into the DOM at some stage, but if you dont want to place it inline with the text, you could absolutely position div or img tags just below the text. Im not sure how you would find the position of the text though (outside of IE which has some non standard methods with TextRanges - see the spellayt plugin for details).
You may be interested in this Highlighting plugin for jQuery though, it pretty much does this.
You can change the class of the container of the text to add highlighting. That container could be the <P> tag that surrounds the text. This will be the entire paragraph, not just the text within the paragraph where the text is found.
Could you provide us with the markup-inclusive text sample, so we can better understand and answer you?