I am converting a legacy system designed for IE6 to work on modern browsers.
Those parts of the site which have not yet been converted, will only work on Internet Explorer, and the IE6 emulation is provided via the following tag.
<meta http-equiv="X-UA-Compatible" content="IE=5">
I am planning on adding HSTS to the site (via CloudFlare). I see that HSTS is supported only for Internet Explorer 11 and higher.
Will adding HSTS cause those parts of my site which rely on IE6 emulation to break?
X-UA-Compatibilty is mainly about HTML rendering of content designed for older versions of IE in newer IE versions. It has by itself nothing to do with HSTS. Adding HSTS probably will be interpreted by the browser even when the HTML content asks for compatibility mode - because HSTS is about the connection (transport of HTML data) and compatibility mode is about the content (interpretation of HTML data). But as long as the site worked before without problems with HTTPS then the added HSTS will not cause additional problems.