The process of creating valid code can sometimes be very difficult, with some websites containing lots of functionality, so making it very time-consuming to error check every single coded element. It is possible to leave websites with un-valid code but inevitably this may mean website users can not use your website the way you intend, or potentially not see your website at all.
A large amount of web-sites both personal and corporate do have un-valid code, and it is quite common to see a homepage error free but to have subpages that are not valid; This is most common when the website contains multiple subpages potentially running content dynamically onto a page template.
It is important when working with any website to create error free source code for your templates, often errors are simple to fix, such as missing missing end tags "/>" and the correct use of "&" but an error in a template design will mean all your pages contain the same problems.
Errors like these are sometimes missed when there is little time for web-site testing or testing on multiple browsers, but do consider when a potential customer comes to your web-site they may not be using the same browser or type of computer as you, and the code that may seem OK on your PC could well read very differently on another PC, these differences do happen and are much more common if code contains errors.
Validating code is best not left until the last moment before the website goes live, doing this could leave you with hundreds of errors to fix at the same time, so do try to make testing an integral part of your coding process. Do also take extra care when creating dynamic pages that will import code from external scripts, as a general rule the more complicated the code the more likely it will cause problems, some of the best websites I have seen are often the most simple.
It is also difficult to keep on top of future changes to websites and quite common for code to be valid, only for future edits to make changes to the website that may contain errors, it is there before important with each edit to go through debugging and error checking process.
It is best practice to make all code error free no matter how slight the error, as web developers we must uphold our duty to create websites for people to view now and in the future without problems.
I do have sympathy for website managers who can not find the time to validate code because I do know it is a slow process, but I think to avoid any display problems coming from errors we owe it to internet users to provide them with validated code, and do not forget if potential customers are viewing your website it is important to show them a website that works well as well as showing them your coding skills are the best.
