I somewhat share EmptyNest's enthusiasm about WordPress, but it surely isn't a "must have".
WordPress is a popular CMS (content management system), and it is great for independent properties. However, you shouldn't discard other CMS's as well, such as Drupal, Joomla, Typo3, etc.
The system itself won't solve your problems, but your website developer will. If he chooses to use WordPress - fine, if he choose Drupal - fine as well.
Think of it this way: even if I buy a Ferrari, if I don't know how to drive - the car is useless, it won't do wonders. Same thing about WordPress - if you implement it badly - it will stink.
Another thing to consider is the available templates/themes. The marketplace of themes for WordPress is huge, you have a wide selection and at very acceptable prices.
A good selection of themes can be found here:
http://www.premiumwp.com/category/premium-themes/hotel-wordpress-themes/
There are less templates for other CMSs, which is one of the main reasons people go with WordPress.
Also, the learning curve with WordPress is much easier than with other CMSs, so there's that.
And I'm also not quite sure why Happykeeper and EmptyNest would compare WordPress and HTML5/CSS. The first is a CMS, the second is a mark-up language, the third is a style sheet language.
A WordPress theme can be coded with HTML5 mark-up and all websites rely on CSS. The same is valid for any other CMS. So don't think of it as "or or"..