On a website which breaks using IE9 but works fine if it’s in IE8 mode (tested it with the developer tools) I’ve added <meta http-equiv="X-UA-Compatible" content="IE=8" /> to the <head> section.
However, it still renders in IE9 mode and the IE9-incompatible JavaScripts (a very old jQuery version and a similarly old jquery ui draggable/droppable version which I can’t simply upgrade for reasons that do not matter here) don’t work.
Do you have any ideas what could be the reason for the meta tag not being honored? According to dev tools IE9 is still in IE9 mode.
there are several sources the IE uses to determine whether to switch to compatibility mode or not:
display in CM
See this official ms site, section “Controlling DefaultRendering”