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When you design website you should always keep in your mind that website follows standards and it is compatible with all browsers, devices such as mobile phones, tablets and so on.
The best way to realize that is your website built based on standards is to check or validate it via W3.org website. This website recognizes that is your website follows standards or not. The good point is that if your website has some error(s) for example missing close tag in HTML, it tells you about error(s) and with correcting errors you again can check the validation of your website. When your website is validated by W3, you can get HTML code to put in your website which indicates to your customers (visitors) that your website follows all W3 standards.
Additionally, W3 checks other validates such as CSS validity, RSS validity, etc. For example, this website has RSS and CSS validates and those signs are shown in the down right of the website.
For checking markup validity please click here.
In order to check CSS validity and RSS validity of your website please click here and here ,respectively.
Finally, this website is also available for checking robots.txt of your website but it is not related to W3 and it is an independent website. The sign of robots.txt validity can be placed on your website.
For more information please visit W3.org website.
