Conditional comments are conditional statements interpreted by Microsoft Internet Explorer in HTML source code.
Conditional comments only work in IE, and are thus excellently suited to give special instructions meant only for IE.
Conditional comments in HTML first appeared in Microsoft’s Internet Explorer 5 browser, although support has now been deprecated. Older IE versions frequently need some extra CSS in order to show your pages right. Conditional comments are the best way to add this CSS, since the system is explicitly designed for this use case. In Internet Explorer 10 HTML conditional comments are not supported when the page is in standards mode (document mode 10). JScript conditional comments were introduced in Internet Explorer 4 and they continue to be supported in Internet Explorer 10, in standards mode or compatibility mode.
If you need special styles for IE10 or up you’ll have to find another method, since conditional comments were disabled in IE10. However, these versions are much less buggy than the earlier ones.