Hello,
OptimusCrime is correct, the problem is with your code and not MODX itself. That being said looking at your website a few suggestions. Consider switching to Bootstrap 3 as the code is more refined, not to mention better responsive breakpoints. This will also save loading the bootstrap responsive css file so one fewer browser request and less page overhead.
You should merge CSS/JS files where possible to eliminate browser requests, keep in mind one larger file loads faster than lots of smaller files. I generally put my framework file standalone followed by my customization css file which contains my overrides followed by any other scripts (ex: prettify). This allows me to update the framework quickly if required, and keep my customizations separate.
You should keep using a CDN for your common scripts where possible, personally I like cdnjs.com (
http://cdnjs.com/) over Google as it's faster in addition to offering more scripts.
Twitter Bootstrap:
http://cdnjs.com/libraries/twitter-bootstrap
jQuery:
http://cdnjs.com/libraries/jquery
The use of the CDN resources also helps with parallel downloads, basically leveraging our super fast connections better.
Getting back to your original question/problem the font issues as noted earlier are related to your code. In addition to including three fonts (not recommended) you aren't including all of the formats as required for proper functionality across devices. I would recommend keeping to two fonts (one is better) to minimize load time, and where possible leverage CDN fonts (aka Google Fonts). If you can't find a Google Font that works for your purposes then use the Font Squirrel Generator (
http://www.fontsquirrel.com/tools/webfont-generator) to generate all of the nessicary font files and CSS for inclusion.
Oh, and if you haven't already make sure gzip (
http://salscode.com/tutorials/2009/10/15/gzip-htaccess/) and expires (see ht.access file from MODX zip) are enabled for your resources.