HTML pages must follow a set of rules to ensure that they can be read properly by web browsers. The process of verifying whether a page follows those rules is called validation, and the tool used for that is a validator. HTML pages created by Second Site should be valid, i.e., a validator should not find any errors when it inspects the page.
Web browsers attempt to recover from HTML errors, and some HTML that is not formally valid is used so frequently that browser authors have added logic to produce good results despite those errors. Even though web browsers are somewhat tolerant of HTML errors, it is a bad idea to rely on the web browser software to do the right thing. The specifics of how one browser reacts to an HTML error can be very different from how a different browser reacts to the same error. That means that a person viewing your site using Internet Explorer may see something different than someone using Firefox. In some cases, the page may look fine on one browser but may not display at all in another.
If your page does not look the way you expect, and especially, if the structure of the page is broken, that may be the result of an HTML error. In that situation, it is a good idea to validate the HTML to determine whether there are any errors. You may use the Markup Validation Service of the W3C to validate HTML pages.
Second Site writes valid HTML, but if you validate a page on your site you may still see validation warnings or errors. Typically, that will occur if you have provided HTML content that is not valid: Second Site mixes HTML that it writes with content that you provide. If you have copied HTML from a third-party source, perhaps to add a search tool, counter, or banner advertisement, the HTML they provided may not be valid. Review the validator output and try to determine what has caused the error.
It is useful to know that a validator will sometimes produce many error messages when there is only a single HTML error. If the validator issues a message that claims some HTML is invalid, but the HTML looks correct to you, it may be a case that some previous HTML has an error that is causing a domino effect. When that happens, fix any obvious errors and then revalidate the page.
Like the HTML on a web page, the CSS statements used by a web page must follow a set of rules, and CSS can be validated in much the same way as HTML.
Second Site writes valid CSS with one exception that is noted below. Also, if you have provided your own CSS statements via User Styles, you may introduce an error.
If your page does not look the way you expect, and especially, if the appearance of the page is broken, that may be the result of a CSS error. In that situation, it is a good idea to validate the CSS to determine whether there are any errors. You may use the CSS Validation Service of the W3C to validate CSS statements.
When you validate CSS written by Second Site v3.2 or greater, the validator may issue error messages for multiple statements related to CSS3 parameters. There are two issues with the statements.
- Second Site uses vendor-specific extensions to implement provisional CSS features in Firefox, Chrome, and Safari. Statement prefixes are -moz for Firefox and -webkit for Chrome and Safari. Validators treat vendor-specific extensions as errors even though the CSS standard explicitly allows their use.
- Second Site uses CSS3 parameters. Most validators default to CSS 2.1, and CSS3 parameters are not valid in CSS 2.1. Specifying CSS3 validation will avoid those warning messages.
Please ignore the validator error messages described above. The parameters will work in the browsers that support them and they will be ignored by other browsers.
This page last changed on 31 Jan 2014.