HTML properties without quotation marks?

Answer 1

Always use quotation marks.

I don't believe that HTML properties without quotation marks are classed as Invalid HTML, but they will potentially cause you problems later on down the line.

By default, SGML requires that all attribute values be delimited using either double quotation marks (ASCII decimal 34) or single quotation marks (ASCII decimal 39). Single quote marks can be included within the attribute value when the value is delimited by double quote marks, and vice versa. Authors may also use numeric character references to represent double quotes (") and single quotes ('). For double quotes authors can also use the character entity reference ".

In certain cases, authors may specify the value of an attribute without any quotation marks. The attribute value may only contain letters (a-z and A-Z), digits (0-9), hyphens (ASCII decimal 45), periods (ASCII decimal 46), underscores (ASCII decimal 95), and colons (ASCII decimal 58). We recommend using quotation marks even when it is possible to eliminate them.

Source: http://www.w3.org/TR/REC-html40/intro/sgmltut.html#h-3.2.2

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I think they're a great way of clearly defining when an attribute value starts and finishes.

Take for example the class attribute which can have multiple classes seperated by spaces:

This clearly shows the browser that my classes are classa and classb, with an element id of 123.

Take away the quotation marks and we've got:

A browser could now interpret this as 3 classes, with no id. classa, classb and id=123.

Or it may even interpret it as 3 attributes. class="classa", classb="" and id="123"

(Even stackoverflow's syntax styling is struggling with this one!)