Clarify strict/lenient mode

Author: sabberworm

README.md

@@ -43,7 +43,7 @@ $parser = new \Sabberworm\CSS\Parser($css, $settings);
 
 #### Strict parsing
 
-To have the parser choke on invalid rules, supply a thusly configured `\Sabberworm\CSS\Settings` object:
+To have the parser throw an exception when encountering invalid/unknown constructs (as opposed to trying to ignore them and carry on parsing), supply a thusly configured `\Sabberworm\CSS\Settings` object:
 
 ```php
 $parser = new \Sabberworm\CSS\Parser(
@@ -52,6 +52,8 @@ $parser = new \Sabberworm\CSS\Parser(
 );
 ```
 
+Note that this will also disable a workaround for parsing the unquoted variant of the legacy IE-specific `filter` rule.
+
 #### Disable multibyte functions
 
 To achieve faster parsing, you can choose to have PHP-CSS-Parser use regular string functions instead of `mb_*` functions. This should work fine in most cases, even for UTF-8 files, as all the multibyte characters are in string literals. Still it’s not recommended using this with input you have no control over as it’s not thoroughly covered by test cases.