Tip: Convert from HTML to XML with HTML Tidy

Listing 1. index.html (an excerpt)

Tidying Up

Obviously, the first step is to download and install HTML Tidy. HTML Tidy is available on most platforms, including Windows, Linux, and MacOS. The default executable is a command-line tool, but GUI versions are available for Windows and MacOS.

To run HTML Tidy, open a terminal and issue the following command:

That's it! HTML Tidy immediately converts index.html into index.xml. HTML Tidy will print messages that highlight issues with the original HTML document during the conversion. In most cases, you can safely ignore these messages.

HTML Tidy runs as a filter, so it expects standard input and it prints the result to the standard output. The redirection operators (< and >) allow you to work with files. By default, HTML Tidy produces a clean HTML page, but you can set two options to output XML, instead:

-asxhtml outputs XHTML documents instead of HTML.

-numeric uses character entities instead of HTML entities. For example, î is replaced with î.

The difference between XHTML and HTML might sound trivial (it's only an extra "X" after all) but it is important. XHTML is a version of HTML 4.01 that has been adapted to the XML syntax. The vocabulary is unchanged (XHTML uses the familiar <p>, <b>, and <a> tags, for example), but the syntax is XML, so it merges nicely in an XML workflow.

The main differences between HTML and XHTML are:

XML elements must have opening and closing tags. HTML does not require the closing tag for many elements, such as <p> unless they are empty elements.

Empty elements follow the XML convention. For example, the line break is written as <br /> instead of <br>.

Attribute values are always quoted (for example, <a href="http://www.marchal.com"> instead of <a href=http://www.marchal.com>).

Listing 2 is the file that HTML Tidy produces when Listing 1 is provided as input. As you can see, it is a valid XML document, and it takes surprisingly little work to produce it.