What tags are forbidden in HTML5?

There are no "forbidden" tags in HTML5 in the absolute sense. However, there are deprecated and non-recommended tags that have either lost their meaning or have been replaced by more semantically accurate and manageable means (for example, through CSS). Using such tags can lead to issues with validation, accessibility, and behavior across different browsers.

Here are some of these tags:

• <acronym>: replaced by <abbr>.
• <applet>: replaced by <object>.
• <basefont>: font management is now fully handled via CSS.
• <big>: deprecated, font size controlled via CSS (font-size).
• <center>: deprecated, alignment controlled via CSS (text-align, margin).
• <dir>: deprecated, replaced by <ul>.
• <font>: deprecated, font management via CSS.
• <frame>, <frameset>, <noframes>: outdated frame model, not recommended. Instead, use <iframe> or other techniques (e.g., SPA, AJAX).
• <isindex>: deprecated.
• <listing>: deprecated, replaced by <pre>.
• <nobr>: deprecated, use CSS (white-space: nowrap).
• <plaintext>: deprecated, replaced by <pre>.
• <s>: deprecated, use <del> or text-decoration in CSS.
• <strike>: deprecated, use <del> or text-decoration in CSS.
• <tt>: deprecated, use <kbd>, <code>, or CSS (font-family: monospace).
• <u>: deprecated, use <ins> or text-decoration in CSS.
• <xmp>: deprecated, replaced by <pre>.

It is important to note that most modern browsers will still parse and display these tags in compatibility mode, but this does not guarantee predictable behavior and does not conform to standards.