diff --git a/docs/src/dictionary/en-custom.txt b/docs/src/dictionary/en-custom.txt
index d4ca484b..cc9c6111 100644
--- a/docs/src/dictionary/en-custom.txt
+++ b/docs/src/dictionary/en-custom.txt
@@ -49,6 +49,7 @@ multiline
 namespace
 namespaces
 newline
+parser's
 parsers
 pre
 prerelease
diff --git a/docs/src/markdown/about/changelog.md b/docs/src/markdown/about/changelog.md
index 5dcbf722..76d9ead7 100644
--- a/docs/src/markdown/about/changelog.md
+++ b/docs/src/markdown/about/changelog.md
@@ -7,7 +7,7 @@
 for which the element under consideration applies.
 - **FIX**: HTML pseudo-classes will check that all key elements checked are in the XHTML namespace (HTML parsers that do
 not provide namespaces will assume the XHTML namespace).
-- **FIX**: Ensure that all pseudo-classes names are case insensitive and allow CSS escapes.
+- **FIX**: Ensure that all pseudo-class names are case insensitive and allow CSS escapes.
 
 ## 1.9.0
 
diff --git a/docs/src/markdown/faq.md b/docs/src/markdown/faq.md
new file mode 100644
index 00000000..252e8572
--- /dev/null
+++ b/docs/src/markdown/faq.md
@@ -0,0 +1,41 @@
+# Frequent Asked Questions
+
+## Why do selectors not work the same in Beautiful Soup 4.7+?
+
+Soup Sieve is the official CSS selector library in Beautiful Soup 4.7+, and with this change, Soup Sieve introduces a
+number of changes that break some of the expected behaviors that existed in versions prior to 4.7.
+
+In short, Soup Sieve follows the CSS specifications fairly close, and this broke a number of non-standard behaviors.
+These non-standard behaviors were not allowed according to the CSS specifications. Soup Sieve has no intentions of
+bringing back these behaviors.
+
+For more details on specific changes, and the reasoning why a specific change is considered a good change, or simply a
+feature that Soup Sieve cannot/will not support, see [Beautiful Soup Differences](./differences.md).
+
+## How does `iframe` handling work?
+
+In web browsers, CSS selectors do not usually select content inside an `iframe` element if the selector is called on an
+element outside of the `iframe`. Each HTML document is usually encapsulated and CSS selector leakage across this
+`iframe` boundary is usually prevented.
+
+In it's current iteration, Soup Sieve is not aware of the origin of the documents in the `iframe`, and Soup Sieve will
+not prevent selectors from crossing these boundaries. Soup Sieve is not used to style documents, but to scrape
+documents. For this reason, it seems to be more helpful to allow selector combinators to cross these boundaries.
+
+Soup Sieve isn't entirely unaware of `iframe` elements though. In Soup Sieve 1.9.1, it was noticed that some
+pseudo-classes behaved in unexpected ways without awareness to `iframes`, this was fixed in 1.9.1. Pseudo-classes such
+as [`:default`](./selectors.md#:default), [`:indeterminate`](./selectors.md#:indeterminate), [`:dir()`](
+./selectors.md#:dir), [`:lang()`](./selectors.md#:lang), [`:root`](./selectors.md#:root), and [`:contains()`](
+./selectors.md#:contains) where given awareness of `iframes` to ensure they behaved properly and returned the expected
+elements. This doesn't mean that `select` won't return elements in `iframes`, but it won't allow something like
+`:default` to select a `button` in an `iframe` whose parent `form` is outside the `iframe`. Or better put, a default
+`button` will be evaluated in the context of the document it is in.
+
+With all of this said, if your selectors have issues with `iframes`, it is most likely because `iframes` are handled
+differently by different parsers. `html.parser` will usually parse `iframe` elements as it sees them. `lxml` parser will
+often remove `html` and `body` tags of an `iframe` HTML document. `lxml-xml` will simply ignore the content in a XHTML
+document. And `html5lib` will HTML escape the content of an `iframe` making traversal impossible.
+
+In short, Soup Sieve will return elements from all documents, even `iframes`. But certain pseudo-classes may take into
+consideration the context of the document they are in. But even with all of this, a parser's handling of `iframes` may
+make handling its content difficult if it doesn't parse it as HTML elements, or augments its structure.
diff --git a/mkdocs.yml b/mkdocs.yml
index b73dfe5c..0d03420f 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -27,6 +27,7 @@ nav:
     - Soup Sieve: index.md
     - API: api.md
     - CSS Selectors: selectors.md
+    - F.A.Q.: faq.md
     - Beautiful Soup Differences: differences.md
   - About:
     - Contributing & Support: about/contributing.md
diff --git a/soupsieve/__meta__.py b/soupsieve/__meta__.py
index b34bd201..18f2ea08 100644
--- a/soupsieve/__meta__.py
+++ b/soupsieve/__meta__.py
@@ -186,5 +186,5 @@ def parse_version(ver, pre=False):
     return Version(major, minor, micro, release, pre, post, dev)
 
 
-__version_info__ = Version(1, 9, 1, ".dev")
+__version_info__ = Version(1, 9, 1, "final")
 __version__ = __version_info__._get_canonical()
diff --git a/tests/test_level4/test_nth_child.py b/tests/test_level4/test_nth_child.py
index 719d26b8..d944a86e 100644
--- a/tests/test_level4/test_nth_child.py
+++ b/tests/test_level4/test_nth_child.py
@@ -41,6 +41,13 @@ def test_nth_child_of_s_complex(self):
             flags=util.HTML
         )
 
+        self.assert_selector(
+            self.MARKUP,
+            ":nth-child(2n + 1 OF :is(p, span).test)",
+            ['2', '6', '10'],
+            flags=util.HTML
+        )
+
 
 class TestNthChildQuirks(TestNthChild):
     """Test `nth` child selectors with quirks."""