Are Single Quotes Allowed in Html

Is it correct to use single quotes for HTML attributes?

It's certainly valid to use single quotes (HTML 4.01, section 3.2.2). I haven't noticed such a trend, but perhaps there's some framework that powers web sites you've visited that happens to quote using single quotes.

Single vs Double quotes (' vs )

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.

Is it acceptable to use single quotes around values in HTML attributes?

Yes, that's acceptable. It works in browsers, and it's allowed by the specifications.

The HTML5 spec says:

In the HTML syntax, attributes can be specified in four different ways:

1. empty attribute syntax
2. unquoted attribute-value syntax
3. single-quoted attribute-value syntax
4. double-quoted attribute-value syntax

The HTML4 spec says:

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.

When should I use double or single quotes in JavaScript?

The most likely reason for use of single vs. double in different libraries is programmer preference and/or API consistency. Other than being consistent, use whichever best suits the string.

Using the other type of quote as a literal:

alert('Say "Hello"');
alert("Say 'Hello'");

This can get complicated:

alert("It's \"game\" time.");
alert('It\'s "game" time.');

Another option, new in ECMAScript 6, is template literals which use the backtick character:

alert(`Use "double" and 'single' quotes in the same string`);
alert(`Escape the \` back-tick character and the \${ dollar-brace sequence in a string`);

Template literals offer a clean syntax for: variable interpolation, multi-line strings, and more.

Note that JSON is formally specified to use double quotes, which may be worth considering depending on system requirements.

Are single/double quotes allowed inside HTML attribute values?

Yes, both quotes are allowed in attribute values, but you must HTML-escape the quote you're using as an attribute value delimiter, as well as other HTML-special characters like < and &:

function encodeHTML(s) {
    return s.split('&').join('&').split('<').join('<').split('"').join('"').split("'").join(''');
}

var html= '<label my_attr="'+encodeHTML(attr_value)+'">Text</label>';

However, you are usually much better off not trying to hack a document together from HTML strings. You risk bugs and HTML-injection (leading to cross-site-scripting security holes) every time you forget to escape. Instead, use DOM-style methods like attr(), text() and the construction shortcut:

$('body').append(
    $('<label>', {my_attr: attr_value, text: 'Text'})
);

Are single quotes valid in HTML/XHTML?

Yes, single quotes are valid.

From the XML spec:

[10]   AttValue  ::= '"' ([^<&"] | Reference)* '"'
                  |  "'" ([^<&'] | Reference)* "'"

Are Single Quotes Valid in a Doctype?

Yes, both are valid.

See the SGML spec. At some point while drilling through all links for the doctype declaration, you'll end up at the "system identifier" specification (the parts containing quotes), which is defined as:

( lit , "
system data [45] ,
lit ) | "
( lita ,    '
system data [45] ,
lita )  '

The definition syntax is weird, but it appears that either single or double quotes are allowed (it's similar to the attribute values definition).

A doctype with single quotes also seems to validate just fine.

Single versus double quotation marks in HTML & CSS

Either are valid, but you probably should stick to a certain style guide. For example, Google's style guide suggest using double quotes for HTML and single quotes for CSS. (Although Google Fonts doesn't follow this exactly)

How can I escape a single quote?

You could use HTML entities:

• ' for '
• " for "
• ...

For more, you can take a look at Character entity references in HTML.

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