HTML Checking for Large Sites

The <table> element does not accept a height attribute. Use CSS instead.

The seamless attribute was proposed to be included in the HTML5 spec, but it wasn’t finally accepted, so it’s not a valid attribute for <iframe>.

The width and height attributes on <img> elements expect a digit to specify the dimension in pixels. It should not contain units, letters or percent signs.

You can achieve this using CSS instead, for example:

<!-- Invalid syntax, the height attribute expects only digits -->
<img src="photo.jpg" alt="cat" height="auto" />

<!-- Valid syntax using CSS -->
<img src="photo.jpg" alt="cat" style="height: auto"  />

Learn more:

• MDN: the img element

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 src attribute is a required attribute, so it cannot be blank.

Example:

<iframe src="https://example.com/map.html"></iframe>

Learn more:

• HTML Living Standard: src attribute on iframe

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>

Learn more:

• HTML Living Standard: src attribute on iframe

The <iframe> element, used to embed another document inside the current document, accepts both attributes width and height which must be valid non-negative integers. Percentages are not allowed for these attributes.

Check the iframe spec.

The width and height attributes on <img> and <iframe> elements expect a digit to indicate the number of pixels. Ensure that this attribute contains only digits.

For example:

<!-- This is invalid because width is not a digit -->
<img width="225px" height="100px" alt="cat" src="cat.jpg" />

<!-- This is valid -->
<img width="225" height="100" alt="cat" src="cat.jpg" />

<iframe> tags are used to embed another document within the current document. To indicate the contents of that nested document, you can either use the src attribute with an URL, or directly provide the contents with the srcdoc attribute. Examples:

<!-- This will embed a document by its URL -->
<iframe src="https://example.com"></iframe>

<!-- This will embed the contents provided in the srcdoc attribute -->
<iframe srcdoc="<p>some content</p>"></iframe>

<!-- This is invalid -->
<iframe><p>some content</p></iframe>