A flaw in your code's logic, such as a failure to validate user input before processing it
Conditions beyond your control, such as a network error on the user's computer
Murphy's Law (anything that can go wrong will go wrong)
Technically, syntax errors are usually runtime errors as well, because the errors occur while the browser loads and attempts to execute the code.
When writing an application you want to be informed of every error that occurs so that you can fix it. But when you deploy your app, the last thing you want users to see is an error message that means nothing to them. Bugs will occur in our software—it's a given. But we can do our best to prevent them, and handle them when they do occur, to provide the best user experience we can.
There are a number of things you can do if you want error-free web pages. First, ensure that your HTML markup is up to snuff. Close all elements that need closing, and properly nest elements. Many HTML-validator services are available for free on the Internet. A popular one is the W3C's Markup Validation Service at ...