# Compile Visualforce Successfully

Learn the requirements of compiling Visualforce pages and components.

You can't save your Visualforce pages and components unless they correctly compile. Here's a list of things to watch out for when creating Visualforce pages:

- Verify that your component tags start with the correct namespace identifier such as `apex:`—that is, `apex` followed by a colon.
- Only Visualforce components and HTML elements are supported in API version 20.0 and later. Markup tags with other prefixes such as `soapenv` aren't supported.
- Make sure that every opening quote and bracket has a closing one.
- Verify that the controller or controller extension is named correctly.
- Visualforce pages and components created using Salesforce API version 19.0 or higher must be written as well-formed XML. In general, this means that elements must be correctly nested, non-empty elements must have an end tag, empty elements must be terminated with a closing slash (“`/`”), and so on. The World Wide Web Consortium (W3C) provides [an article on the specifications of well-formed XML](http://www.w3.org/TR/xhtml1/#diffs).

  The following exceptions are allowed:

  - Code that violates well-formed XML is permitted inside JavaScript. For example, you don't need to use `<![CDATA[]]>` tags in Visualforce.
  - Code that violates well-formed XML is permitted inside expressions. For example, you don't need to escape quotation marks inside formulas.
  - XML directives that are normally required at the beginning of a page—such as `<?xml version="1.0" encoding="UTF-8"?>`—can occur inside top-level container tags, like `<apex:page>` and `<apex:component>`.