Referring to LESS documentation (http://lesscss.org/#client-side-usage):
Link your .less stylesheets with the rel set to “stylesheet/less”:
<link rel="stylesheet/less" type="text/css" href="https://stackoverflow.com/questions/18335299/styles.less" />
Then download less.js from the top of the page, and include it in the <head> element of your page, like so:
<script src="https://stackoverflow.com/questions/18335299/less.js" type="text/javascript"></script>
Make sure you include your stylesheets before the script.
This method will cause the less-file act as a regular css-file so no additional files are needed.
This is not a good practice for production environments though. You should compile your less-file to css-file and include it in your <head>-section as is without any less- or js-files.