HTML Guide
The specified language code in the lang attribute of the <html> tag is not a valid ISO code.
The media attribute on a <link> element has not been recognized.
This attribute specified what media the linked resource is optimized for. As an example, the following will link a general stylesheet, and a specific one for printing:
<head>
<link rel="stylesheet" type="text/css" href="general.css">
<link rel="stylesheet" type="text/css" href="print.css" media="print">
</head>Valid values for this attribute include:
- all. Default, used for all media.
- print. Used for printers and print previews.
- screen. Used for computer, tablets or smartphone screens.
The value of the name attribute on an <iframe> should not start with an underscore (_).
Browsing context names that begin with an underscore are reserved keywords in HTML, like _blank, _self, _parent, and _top. Using these reserved names or any custom name starting with an underscore for the name attribute of an <iframe> can lead to unexpected behavior and is considered invalid HTML.
Here’s how to fix the issue:
Problematic Code
<iframe src="https://example.com" name="_example"></iframe>Solution
To resolve this issue, you should use a valid value for the name attribute that does not start with an underscore.
Corrected Code
<iframe src="https://example.com" name="example"></iframe>Steps:
- Identify the iframe element with the invalid name attribute value that starts with an underscore.
- Replace the name value with a valid identifier that does not start with _. Use letters, numbers, hyphens (-), and underscores (_) (but not at the beginning).
The novalidate attribute is boolean: the presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value. As a boolean attribute, it does not need to be passed any value such as true or 1 to activate the novalidate property.
This boolean attribute indicates that the form shouldn’t be validated when submitted. If this attribute is not set (and therefore the form is validated), it can be overridden by a formnovalidate attribute on a <button>, <input type="submit">, or <input type="image"> element belonging to the form.
Example:
<form method="post" novalidate>
<label>User Name:
<input name="user-name" autocomplete="name">
</label>
<button>Save</button>
</form>The W3C Validator issue you’re seeing is because the ping attribute on the <a> (anchor) element is supposed to contain a space-separated list of URLs to which, when the hyperlink is activated, the browser will send ping requests. The ping attribute must only contain proper “http” or “https” URLs, that is, relative URLs are not allowed.
Example of Incorrect Code
<a href="https://example.com" ping="/track">Visit Example</a>Examples of Correct Code
-
Proper Ping URL: Ensure the ping URL starts with http or https and is correctly formatted.
<a href="https://example.com" ping="https://example.com/track">Visit Example</a> -
Multiple URLs: If you have multiple URLs, they should be space-separated and each should be a valid URL.
<a href="https://example.com" ping="https://example.com/track https://example.com/another-track">Visit Example</a>
Common Mistakes to Avoid
- Using invalid URL schemes like mailto: or /relative-path.
- Including broken URLs or URLs with protocols other than http or https.
- Incorrectly formatting multiple URLs; they must be space-separated, not comma-separated or otherwise.
Note
The ping property is not effective in Firefox and its usage may be limited due to privacy and security concerns.
The rel attribute defines the relationship between a linked resource and the current document. Valid on <link>, <a>, <area>, and <form>, the supported values depend on the element on which the attribute is found.
Here’s an example of using the rel attribute to link a document to a CSS stylesheet:
<link rel="stylesheet" href="default.css" />Here’s an example os using the rel attribute to tell search engine spiders to ignore the link relationship with another document:
<a href="https://example.com" rel="nofollow">more info</a>The boolean required attribute, if present, indicates that the user must specify a value for the input before the owning form can be submitted.
If this property is not present, the input will be considered as optional. To mark an input as required, it’s enough to include the property without any value, or pass it the required value as in these examples:
Example:
<input type="text" required>
<input type="text" required="required">The sandbox attribute is used with the iframe element to isolate the content of the embedded document from the rest of the page. It helps prevent malicious code from running on your website. However, the value assigned to the sandbox attribute in your iframe element includes both the allow-scripts and allow-same-origin options. This combination essentially removes all the protections that the sandbox attribute provides and allows the embedded document to break out of the sandbox.
To fix this issue, you should remove the allow-scripts and allow-same-origin values from the sandbox attribute. Instead, you should explicitly enable only the permissions that the embedded document requires.
Here’s an example iframe element with the proper use of sandbox:
<iframe src="https://example.com" sandbox="allow-forms allow-popups"></iframe>This iframe element loads the https://example.com URL and has its sandbox attribute set to only allow-forms and allow-popups. This explicitly enables only the permissions that the embedded document may need, while also retaining the protections of the sandbox attribute.
An <iframe> element allows to embed an HTML document inside another HTML document, and its src attribute is indicated the source URL of the embedded web page. The query part of that URL contains one or more space characters, which are not allowed, for example:
<iframe src="https://maps.google.it/maps?q=2700 6th Avenue"></iframe>You should properly escape all space characters as %20 like this:
<iframe src="https://maps.google.it/maps?q=2700%206th%20Avenue"></iframe>This error message indicates that there is a backslash (\) used in a URL, which is not a valid character for URL paths.
You’ll need to replace the backslashes with forward slashes (/) in the URL path segments.
Here’s an example of a correct img tag using a valid URL path:
<img src="https://example.com/img/small/photo.png" alt="example image">Also, make sure that the URL is correct and that the image file actually exists in the specified location.
The src attribute on an element <img> contains a character which is not allowed unless properly encoded.
Special characters needing encoding are: :, /, ?, #, [, ], @, !, $, &, ', (, ), *, +, ,, ;, =, as well as % itself.
For example, this image tag is incorrect because the src attribute contains an the unallowed characters [ and ]:
<img src="image[00].svg" alt="logo">Instead, this is the properly percent-encoded src attribute, where [ has been replaced with %5B and ] with %5D.
<img src="image%5B00%5D.svg" alt="logo">Space characters are not allowed in src attributes. Instead, they should be converted to %20. In this example, the first line is invalid and the second is valid:
<img src="https://example.com/?s=some term" alt="description" />
<img src="https://example.com/?s=some%20term" alt="description" />The src attribute on an element <img> contains a character that is not allowed, and should be encoded.
Some typical examples include the pipe character | that should be replaced by its encoded alternative %7C , and the left square bracket [ that needs to be encoded as %5B.
The src attribute on an <img> tag is not allowed to contain space characters. You should replace them with “%20“.
URLs need to be encoded so that special characters area escaped, for example space characters need to be converted to “%20”. All special characters will therefore be converted to a percent sign followed by two hexadecimal characters, to be later decoded. In case a percentage sign (%) is found without being followed by two hexadecimal digits, a decoding error will be raised.
The most probable cause is an URL that contains a % that has not been properly encoded to %25 which is the code for the percent sign. For example:
<!-- This is invalid as the percentage sign has not been properly encoded -->
<img alt="" src="https://example.com/img.jpg?width=48%" />
<!-- This is valid as the percentage sign has been encoded as %25 -->
<img alt="" src="https://example.com/img.jpg?width=48%25" />To fix the issue of having a bad value for the src attribute on an img element due to having a tab, new line, or carriage return, you need to ensure that the src attribute does not contain any additional whitespace characters like tab, new line, or carriage return.
Incorrect code:
<img src="images/example.jpg
" alt="Example Image">Corrected code:
<img src="images/example.jpg" alt="Example Image">By removing the extra whitespace characters (new line in this case) and ensuring that the src attribute value is properly enclosed within quotes, the issue should be resolved.
The src attribute value is malformed, check that it doesn’t contain extraneous characters.
For example, this is invalid because the URL in the src attribute contains a space in between the https: and // parts:
<a href="https: //example.com">Some link</a>To fix this issue, you can try removing the space after “https:” in the URL in the src attribute:
<a href="https://example.com">Some link</a>The srcset property on img elements, when used, requires at least one value. This property is a string which identifies one or more image candidate strings, separated using commas (,) each specifying image resources to use under given circumstances.
Each image candidate string contains an image URL and an optional width or pixel density descriptor that indicates the conditions under which that candidate should be used instead of the image specified by the src property.
Example:
<img
src="/img/cat-200px.png"
alt="Cat"
srcset="
/img/cat-200px.png 1x,
/img/cat-400px.png 2x
">URLs need to be encoded so that special characters area escaped, for example space characters need to be converted to “%20”. All special characters will therefore be converted to a percent sign followed by two hexadecimal characters, to be later decoded. In case a percentage sign (%) is found without being followed by two hexadecimal digits, a decoding error will be raised.
The most probable cause is an URL that contains a % that has not been properly encoded to %25 which is the code for the percent sign. For example:
<!-- This is invalid as the percentage sign has not been properly encoded -->
<img alt="" src="https://example.com/img.jpg?width=48%" />
<!-- This is valid as the percentage sign has been encoded as %25 -->
<img alt="" src="https://example.com/img.jpg?width=48%25" />The srcset property on source elements, when used, requires at least one value.
The <source> HTML element specifies multiple media resources for the <picture>, the <audio> element, or the <video> element. It is a void element, meaning that it has no content and does not have a closing tag. It is commonly used to offer the same media content in multiple file formats in order to provide compatibility with a broad range of browsers given their differing support for image file formats and media file formats.
The srcset attribute is required if the source element’s parent is a <picture> element, but not allowed if the source element’s parent is an <audio> or <video> element.
The sizes attribute is used to complement the srcset attribute on an <img> tag for responsive images. When this attribute is present, all image candidates must specify its width.
The value used in the target property of an <a> element has been identified as a keyword because it starts with an underscore _, but it’s not a valid keyword.
Values starting with an underscore for the target property are used for reserved keywords like _blank, _self, _parent or _top.
The value provided on the type attribute of an a element is not a valid MIME type.
The type attribute expects a MIME type that hints at the linked URL’s format.
The type attribute on <a> elements, when present, gives a hint on the MIME type of the linked resource, for example:
<a href="application/pdf" src="book.pdf">Read our book</a>
<a href="image/jpeg" src="photo.jpeg">See a photo</a>That is, we’re talking about the type of the linked resource, not the type of the <a> element, as it’s sometimes misunderstood. The following example is invalid because button is not a valid MIME type.
<a href="/order.php" type="button">Submit</a>The value used to define the type of a link is not valid. You’re probably using a URL instead of a valid type.
Example of a valid type:
<link rel="icon" type="image/png" href="favicon.png">