There are many ways to create a website but in general you can separate it in two categories. We have the markup code that is generated in a dynamic way from server side languages (like php and python). and markup that is specified in the files you include, this is the static method with hard-coded markup.
The first case is really hard to check, correct and edit, while in the second place we have a simple find & replace but how many times you should repeat this action depends on the repetitiveness of the code.
The most famous standards kept are the w3c ones, and so it happens that they provide tools for the validation of your markup. The main links for these validators are:
– validator.w3c.org for html and xml validation
–css validator for css validation
In both cases you get specific errors and their occurrences, but its up to you to decide if these rules should be kept, as in most cases these changes and standards will make no difference to the modern browsers.
Why do you need to validate your code, two are the main reasons:
-> In this manner you will be ensured that your webpage will appear correctly on most browsers and the way you want it to be
-> Mistakes in markup language syntax may greatly increase the waste of your resources and may even cause page slowdowns
Ultimately the decision is up to you. You should balance and compare the time it needs to be fulfilled and the difference it will make.
For your information: I try to perfect my websites and optimize them only from sheer passion.