There are several coding rules required for XHTML that were optional or even unsupported in HTML 4. While HTML5 supports both coding formats, here are some non-required best practices:
- Use lowercase for markup: In XHTML the element tags must all be in lowercase as must all the attribute names. While HTML5 supports camel case and everything else, please use all lowercase markup. Nothing in the W3C states that attribute values need to be lowercase, but some, like id, are case sensitive, so stick to lower case.
- Quote all attributes: In XHTML, all attribute values must be encased in single or double quotes. In HTML5, only attribute values with spaces or special characters are required to be in quotes. Make your code legible. Make us happy. Quote all attributes.
- Close all elements: In XHTML, every opening tag must have a closing tag. Empty elements such as <img/> and <br/> must be self-closing. In HTML5 some tags can be left unclosed. While omitting the trailing slash or closing element reduces the number of characters on a page, it also allows for sloppy code.Some speakers / blog posts recommend omitting trailing slashes and end tags to reduce the number of characters to make smaller files for mobile. The few bytes saved are not worth the risk, especially in non-empty elements. Reducing the number of DOM elements will have more of an impact than reducing the number of characters in this way. Make us happy. Provide a closing tag for each opening tag.
- Nest all elements: In XHTML, all tags must be properly nested: If you start tag <a> and then start, or nest, a <strong>, you must close tag </strong> before you close the </a>. All tags must be nested correctly, which is easier to trouble shoot if you close all elements (see above).
- Be consistent in providing values for all attributes: In XHTML, all attributes must be coded as attribute/value pairs, even Boolean values. The default selected option in XHTML should be written selected=”selected”. In HTML5, the same can simply be coded as selected. Providing values for Boolean attributes is not necessary, since including the attribute property makes a Boolean value true even if the value of the attribute is false in most browsers. Whether you choose to include or not include Boolean values, be consistent with your decision either way. If you include Boolean values, always include them. If you omit them, always omit them. And, if you include a =”” for a Boolean (or even non-Boolean) attribute, always include the value, as the empty quotes can have unintended consequences, especially when it comes to the form attribute.
- Use the most semantically correct element for the job: In XHTML, elements need to be coded in a semantic manner. Tables and forms cannot be included in paragraphs. Form elements, being phrase elements, need to be contained within a semantic block level element, such as a paragraph or table cell. Yes, you can use spans and divs to contain everything on your page, but if a header is a header, use an <h1-6> tag.