Top 10 HTML Warnings in Portugal

The aria-hidden attribute is redundant when the hidden attribute is already present on an element.

Trailing slashes on void elements like <br/>, <img/>, or <hr/> serve no purpose in HTML and are ignored by browsers. However, they can cause subtle bugs when combined with unquoted attribute values, because the slash may be parsed as part of the last attribute's value rather than as a self-closing indicator. The simplest fix is to remove trailing slashes from all void elements.

Since HTML5, the default scripting language for <script> elements is JavaScript, making type="text/javascript" redundant. The W3C validator flags this as an unnecessary attribute. Simply remove the type attribute from any <script> tag that loads or contains JavaScript.

Duplicate id attributes in an HTML document cause validation errors, accessibility problems, and unpredictable JavaScript behavior. The W3C validator flags this message to point you back to the first element that uses a given id, helping you locate where the duplication begins. To fix it, ensure every id value appears only once per page by renaming or removing the duplicates.

HTML comments containing two consecutive hyphens (--) inside the comment body are not valid according to XML 1.0 rules, which the HTML serialization rules also enforce. To fix this, remove or replace any double hyphens (--) that appear between the opening and closing delimiters of your comments.

The W3C HTML Validator warns when a <section> element does not contain a heading (<h2>–<h6>). The <section> element is intended to represent a thematic grouping of content that typically has its own heading. To fix this, either add a heading element inside each <section>, or consider using a different element like <div> if the content doesn't represent a distinct, named section.

Attribute names containing certain characters — such as curly braces, template syntax markers, or control characters — cannot be represented in XML 1.0 and will trigger this validation error. This commonly happens when template placeholders (e.g., {{variable}}) or special characters are accidentally left in the rendered HTML output. To fix it, ensure all dynamic expressions are properly resolved before the HTML is served, and that attribute names only contain valid characters.

The type attribute on the <style> element is unnecessary in HTML5 and should be removed. Since CSS is the default and only supported styling language for HTML, specifying type="text/css" is redundant. Simply remove the type attribute from your <style> tags to resolve this warning.

The W3C HTML Validator reports this error when an HTML element uses a presentational attribute that was deprecated in HTML5. These legacy attributes — such as align, bgcolor, border, cellpadding, cellspacing, width, height, valign, and others — controlled visual styling directly in HTML but have been replaced by CSS. To fix this, remove the obsolete attribute from the element and apply the equivalent CSS property instead.

The <article> element represents a self-contained composition, but when it lacks a heading (<h2>–<h6>), assistive technologies and users have no way to quickly identify its content. Fix this by adding a descriptive heading element inside each <article> to give it a clear, accessible label.