Is Quoting the Value of Url() Really Necessary

Is quoting the value of url() really necessary?

The W3C says quotes are optional, all three of your ways are legal.

Opening and closing quote just need to be the same character.

If you have special characters in your URL, you should use quotes or escape the characters (see below).

Syntax and basic data types

The format of a URI value is 'url(' followed by optional white space followed by an optional single quote (') or double quote (") character followed by the URI itself, followed by an optional single quote (') or double quote (") character followed by optional white space followed by ')'. The two quote characters must be the same.

Escaping special characters:

Some characters appearing in an unquoted URI, such as parentheses, white space characters, single quotes (') and double quotes ("), must be escaped with a backslash so that the resulting URI value is a URI token: '\(', '\)'.

If a URL contains a quote how do you specify the rel=canonical value?

You have to split your question into two:

Do I need to encode the double quotation mark character in the URL path?

Yes, the quotation mark character (U+0022) is not allowed in plain and must be encoded with %22.

Do I need to encode the double quotation mark character in a HTML attribute value?

It depends on how you declare the attribute value:

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 ".

• If you’re using double quotation mark character to declare the attribute value (attr="value"), then you must encode the douvke quoteation mark character inside the attribute value declaration with a character reference (", " or ").
• If you’re using the single quotation mark character (U+0027) for your attribute value declaration (attr='value'), then you don’t need to encode the quotation mark character. But it’s recommended to do so.

And since you have slash and a double quotation mark in your attribute value, the third case (using no quotes at all) is not applicable:

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.

Now bringing both answers together

Since a double quotation mark must be encoded in a URL (but the single quotation mark is!), you can use the following to do so with the path segments or you URL path:

$path = '/thisisa"quote/helloworld/';
$path = implode('/', array_map('rawurlencode', explode('/', $path)));

And if you want to put that URL path in a HTML attribute, use the htmlspecialchars function to encode remaining special HTML characters:

echo '<link rel="canonical" href="' . htmlspecialchars($path) . '" />';

When should I wrap quotes around a shell variable?

General rule: quote it if it can either be empty or contain spaces (or any whitespace really) or special characters (wildcards). Not quoting strings with spaces often leads to the shell breaking apart a single argument into many.

$? doesn't need quotes since it's a numeric value. Whether $URL needs it depends on what you allow in there and whether you still want an argument if it's empty.

I tend to always quote strings just out of habit since it's safer that way.

CSS background-image - What is the correct usage?

The path can either be full or relative (of course if the image is from another domain it must be full).

You don't need to use quotes in the URI; the syntax can either be:

background-image: url(image.jpg);

Or

background-image: url("image.jpg");

However, from W3:

Some characters appearing in an unquoted URI, such as parentheses, white space characters, single quotes (') and double quotes ("), must be escaped with a backslash so that the resulting URI value is a URI token: '\(', '\)'.

So in instances such as these it is either necessary to use quotes or double quotes, or escape the characters.

Some images not working in `background-image:url()`

Escape special characters (\ in front of ( and )) in the URL.

<!-- Use CSS escape character to escape special characters -->
<div style="background-image:url(https://ignite.galify.com/images/tx9gyzjfb24712-\(1\).jpg)"> img </div>

<!-- Working -->
<div style="background-image:url(https://ignite.galify.com/images/9mspqz9gqb1561725209327568845756.jpg)"> img1 </div>

`/admin/${this.seriesId}/lessons`