Rocket Validator integrates the W3C Validator HTML checker into an automated web crawler.
HTML issues tagged as tt.
The <tt> tag, used in HTML4 to apply a monospaced (fixed width) font to the text, was deprecated in HTML5. Instead, you should use CSS to apply the desired font.
Example, instead of this deprecated code:
<tt>This is deprecated</tt>
You can define a monospaced text using font-family:
<span style="font-family: monospace;">This is monospaced text</span>
When was the last time you validated your whole site?
Keep your sites healthy checking for A11Y/HTML issues on an automated schedule.