Valid XML document
A valid XML document is validated against a Document Type Definition (DTD) or XML Schema. Although it is recommendable, it is not necessary for a document XML to be valid for its successful performance.
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Visit
A visit is a series of requests from the same uniquely identified individual (a visitor) in a set period of time. A visit is made up of several hits and pageviews and often shows a track through a site.
Visits are usually defined as a set period of time, typically 24 hours or 1 hour. It's important to know what the log analysis software defines as a visit period. Visits are calculated by setting a cookie or web bug on the customer, so they can be analysed later, and websites can better adjust to the user’s preferences.
Visitor
A visitor is a unique individual coming to a website. Any visitor can visit one or more pages, so it is interesting to use a register based on our own tools or on professional tools such as Google Analytics. Many Web analytics packages try to identify visitors through IP address and browser/OS information. The number of visitors to your website is an excellent metric regarding how popular your site is.
Void element
A void element is a tag that does not require a closing tag to be valid HTML. The mostcommon void elements are:
<nobr> <img> <input>