Is using a double-forward slash (//) for single-line comments in CSS considered reliable and supported by mainstream browsers and CSS interpreters according to the specification?
Is using a double-forward slash (//) for single-line comments in CSS considered reliable and supported by mainstream browsers and CSS interpreters according to the specification?
When it comes to CSS, comments made using double slashes // are considered invalid. According to the CSS spec, only the following is mentioned about comments:
4.3.2. Consume comments
This section explains how to consume comments from a string of code points, without returning anything.
If the next two input code points are U+002F SOLIDUS (/) followed by a U+002A ASTERISK (*), consume them along with all subsequent code points until the first U+002A ASTERISK (*) followed by a U+002F SOLIDUS (/), or until reaching an EOF (End of File) code point. Repeat this step if necessary.
If the previous paragraph ended by consuming an EOF code point, it signifies a parse error.
No return value specified.
In essence, only /* */ are recognized as valid comments in CSS. The spec does not acknowledge //.
However, in certain CSS processors like Less and SASS, // is considered valid.
In response to your query:
...can you rely on browsers to understand that's a comment
No, most browsers will interpret the syntax regardless and likely deem it as a syntax error rather than recognizing it as a comment. The outcome might vary across browsers, but utilizing it would result in undefined behavior.
The application of the following rules results in varying outcomes across different browsers. One style includes using double slashes at the start of the property, while the other has // right before the value.
#some {
width: 500px;
/*height: 400px;*/
//color: blue;
background-color: //red;
}
https://i.sstatic.net/EjMrL.png
In Firefox ESR 52.9.0, a small yellow warning triangle appears adjacent to color and background-color due to //color being an invalid CSS property, and //red representing an incorrect background-color value.
https://i.sstatic.net/gTjuY.png
Intriguingly, in Chrome 68.0.3440.106, the line //color:blue may not even appear in the elements panel, indicating that Chrome possibly considers it a comment. However, since the recognition of // as comments is not standard, relying on it is not recommended. Furthermore, the warning for background-color persists due to //red being an incorrect value.
https://i.sstatic.net/pVgpW.png
Safari 11.1.2 exhibits similar behavior to Chrome where the properties starting with // are not listed, and the values led by // generate syntax errors.
https://i.sstatic.net/AquCI.png
Internet Explorer 11.0.9600.19080 interprets //color: blue as a rule property lacking a value, akin to //color: blue: ;. It also identifies background-color: //red as erroneous and doesn't execute it.
It is important to mention that in the case of the following:
#some {
//width: 400px;
/* height: 400px; */
}
Most browsers recognize and allow toggling the /* */ property in Developer Tools. In contrast, Chrome and Safari do not list the // led rule, making it impossible to toggle it as with /* */.
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