Single vs Double quotes (' vs ")

Question

The w3 org said:

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 ".

So... seems to be no difference. Only depends on your style.

I use " as a top-tier and ' as a second tier, as I imagine most people do. For example

<a href="#" onclick="alert('Clicked!');">Click Me!</a>

In that example, you must use both, it is unavoidable.

Quoting Conventions for Web Developers

The Short Answer

In HTML the use of single quotes (') and double quotes (") are interchangeable, there is no difference.

But consistency is recommended, therefore we must pick a syntax convention and use it regularly.

The Long Answer

Web Development often consists of many programming languages. HTML, JS, CSS, PHP, ASP, RoR, Python, etc. Because of this we have many syntax conventions for different programming languages. Often habits from one language will follow us to other languages, even if it is not considered "proper" i.e. commenting conventions. Quoting conventions also falls into this category for me.

But I tend to use HTML tightly in conjunction with PHP. And in PHP there is a major difference between single quotes and double quotes. In PHP with double quotes "you can insert variables directly within the text of the string". (scriptingok.com) And when using single quotes "the text appears as it is". (scriptingok.com)

PHP takes longer to process double quoted strings. Since the PHP parser has to read the whole string in advance to detect any variable inside—and concatenate it—it takes longer to process than a single quoted string. (scriptingok.com)

 

Single quotes are easier on the server. Since PHP does not need to read the whole string in advance, the server can work faster and happier. (scriptingok.com)

Other things to consider

1. Frequency of double quotes within string. I find that I need to use double quotes (") within my strings more often than I need to use single quotes (') within strings. To reduce the number of character escapes needed I favor single quote delimiters.
2. It's easier to make a single quote. This is fairly self explanatory but to clarify, why press the SHIFT key more times than you have to.

My Convention

With this understanding of PHP I have set the convention (for myself and the rest of my company) that strings are to be represented as single quotes by default for server optimization. Double quotes are used within the string if a quotes are required such as JavaScript within an attribute, for example:

<button onClick='func("param");'>Press Me</button>

Of course if we are in PHP and want the parser to handle PHP variables within the string we should intentionally use double quotes. $a='Awesome'; $b = "Not $a";

Sources

Single quotes vs Double quotes in PHP. (n.d.). Retrieved November 26, 2014, from http://www.scriptingok.com/tutorial/Single-quotes-vs-double-quotes-in-PHP

If it's all the same, perhaps using single-quotes is better since it doesn't require holding down the shift key. Fewer keystrokes == less chance of repetitive strain injury.

In HTML I don't believe it matters whether you use " or ', but it should be used consistently throughout the document.

My own usage prefers that attributes/html use ", whereas all javascript uses ' instead.

This makes it slightly easier, for me, to read and check. If your use makes more sense for you than mine would, there's no need for change. But, to me, your code would feel messy. It's personal is all.