Rocket Validator automatically checks your pages on the W3C Validator.
HTML issues tagged as windows-1252.
<meta> tag has been found in the document stating that the charset is
windows-1252, but it actually is
utf-8. You should update the tag to reflect the actual encoding of the document, for example:
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
In HTML5 you’re encouraged to use Unicode (UTF-8) character encoding rather than a legacy character encoding such as Latin1 (Windows-1252 or ISO 8859-1).
In short, it can be just a matter of using
<meta charset="utf-8"/> in your document, but you should also ensure that your pages are also saved and served as UTF-8.
When was the last time you validated your whole site?
Keep your sites healthy checking for A11Y/HTML issues on an automated schedule.