1. Increased Loading Time
However, not all visitors to your website will have a fast internet connection. Some may living in remote locations with slow internet connections. Remember dialup? Some internet connections haven’t gotten much faster than that.
2. Increased Waiting Time
However, not everybody uses a developer-level computer. Some people use computers that will require a significant amount of time to even start up a browser.
Having to wait can greatly decrease the quality of these visitors’ experience.
3. Different Implementations
In general, there are commonly agreed upon standards for how portions of code should be executed. However, not every browser agrees to follow all of the rules. Internet Explorer has historically failed to accept web development standards, causing problems for web developers. (But, thankfully for developers, Microsoft has been getting better about complying with them.)
In addition to browser differences, your code will likely need to operate differently depending on each visitors’ individual machine. Windows, Mac OS, Linux, iOS, Android, and other operating systems all have slightly different (or some times very different) ways of displaying website content.