You'd be surprised. The amount of CSS/HTML that goes into your typical LOB application is quite large and the applications are typically orders of magnitude more complicated than "sites" as you mention. The piece of kit I work with has approximately 900 ASP.Net MVC views...
I just ran CLOC. We have 6.2 million lines of code in 14 different languages!
It's orders of magnitude more complicated, and generally for no good reason. There are often conflicting CSS rules, or 100 lines of cruft when 10 lines might work just fine.
And yes, things break all the time, often for unknown or unexpected reasons, and it's sometimes close to impossible to figure out why and fix the bug.
As someone who worked on an internal NHS website I can affirm this. Rather than use the same CSS and HTML for a form no matter which page it appeared on, the original coders had re-written it from scratch every single time, with slight differences (some intentional, some bugs) between them all.
Oh, and the entire site didn't work in IE6, it was developed / tested entirely targeting Firefox, despite the IE6 requirement being in the spec (and it ONLY needing to work with IE6).
Some of the most advanced CSS and Javascript I have seen for the first time in Enterprise apps. I first saw xmlhttprequest in a Cognos controller, I first saw drag+drop in a browser in another enterprise app.
the Outlook web interface was miles ahead of anything in the consumer space for years.
Indeed. We basically wrote our own version of jQuery, backbone.js and a whole functional toolkit with 150 functions and 30 client side controls and a validation framework about 5 years ago and it even worked on IE 5.5...
I just ran CLOC. We have 6.2 million lines of code in 14 different languages!