HTML attribute names are usually case-insensitive, but the specific behavior depends on the attribute type and usage scenario. Native attributes such as id, class, src, etc. are case-sensitive in HTML, and the browser will automatically convert to lowercase parsing, such as CLASS and class equivalents. Custom data attributes (data-) are also case-insensitive, but when accessed through dataset in JavaScript, you need to be named with camels, such as data-userType corresponding to userType. When operating properties in JavaScript, you should note that getAttribute is not case sensitive, while some DOM properties such as className must be accessed in correct case. Attribute values ??are usually case sensitive, especially in scenarios such as URLs and file names. It is recommended to use lowercase attribute names uniformly to avoid problems and follow the data-standard format to facilitate JS operation.
HTML attributes themselves are usually case-insensitive, but their performance depends on the type and usage scenarios of the attribute.

When the browser parses HTML, it will automatically convert the tag name and most attribute names to lowercase, so syntactically, when you write common attributes such as class, id, and src, it is okay to use uppercase or mixed writing methods. For example:

-
<div class="example"> and <code><div class="example"> have the same effect<li> <code>ID="test"
andid="test"
are also equivalent in HTML
However, in some specific cases, upper and lower case affects behavior. The following are some common cases.
Native HTML attributes are generally case-insensitive
Most native properties (such as id, class, href, src, type, etc.) are case-insensitive in HTML. This is because the HTML specification states that these attribute names are "ASCII case-insensitive".

For example:
<img src="/static/imghw/default1.png" data-src="image.jpg" class="lazy" ALT="My Image" alt="Are html attributes case-sensitive?" >
This can display pictures and alternative texts normally in the browser. Although it is in capital, there will be no errors.
However, it should be noted that attribute values ??are usually case sensitive. for example:
<link rel="StyleSheet" href="style.css">
Although rel="StyleSheet"
here can be recognized, it is recommended to use the standard writing method stylesheet
, because some servers or scripts may strictly match the content of the value.
Custom data attributes (data-*) are case-sensitive
Custom attributes such as data-role
or data_user_type
are actually processed in lowercase by the browser and can be accessed through dataset
.
For example:
<div data-userType="admin"></div>
When accessing in JavaScript, name it with camel:
element.dataset.userType; // Get "admin" correctly
But note that if you write a property like data-user-type
, JavaScript will automatically turn it into camel form: userType
.
Pay attention to case issues when operating properties in JavaScript
When you get or set properties through JavaScript, some methods are case sensitive and some are not sensitive.
- Both
getAttribute("class")
andgetAttribute("CLASS")
can get the class name. - However, for some DOM properties, such as
className
, they must be accessed in correct case.
Also, if you are operating on XML or XHTML documents (this is rare now), the attribute name is case sensitive.
Summary of suggestions
To avoid potential problems, we recommend you:
- Unified use of lowercase writing HTML attribute names, which is a common community specification
- Be cautious about property values, especially in cases where URLs, file names, class names, etc. may depend on upper and lower case
- When using custom properties, follow the
data-*
standard format, which is convenient for JS operation
Basically that's it. Although HTML attributes are not case sensitive, keeping consistent writing will make the code clearer and less error-making.
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