diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index e39fe20310c..efdfcafb40a 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -91,6 +91,7 @@ pub mod plugins;
 pub mod visit_ast;
 pub mod visit_lib;
 pub mod test;
+pub mod theme;
 
 use clean::AttributesExt;
 
@@ -267,6 +268,11 @@ pub fn opts() -> Vec<RustcOptGroup> {
                        "additional themes which will be added to the generated docs",
                        "FILES")
         }),
+        unstable("theme-checker", |o| {
+            o.optmulti("", "theme-checker",
+                       "check if given theme is valid",
+                       "FILES")
+        }),
     ]
 }
 
@@ -316,6 +322,27 @@ pub fn main_args(args: &[String]) -> isize {
         return 0;
     }
 
+    let to_check = matches.opt_strs("theme-checker");
+    if !to_check.is_empty() {
+        let pathes = theme::load_css_pathes(include_bytes!("html/static/themes/main.css"));
+        let mut errors = 0;
+
+        println!("rustdoc: [theme-checker] Starting tests!");
+        for theme_file in to_check.iter() {
+            print!(" - Checking \"{}\"...", theme_file);
+            if !theme::test_theme_against(theme_file, &pathes) {
+                eprintln!(" FAILED");
+                errors += 1;
+            } else {
+                println!(" OK");
+            }
+        }
+        if errors != 0 {
+            return 1;
+        }
+        return 0;
+    }
+
     if matches.free.is_empty() {
         print_error("missing file operand");
         return 1;
diff --git a/src/librustdoc/theme.rs b/src/librustdoc/theme.rs
new file mode 100644
index 00000000000..ff1adc3e4c4
--- /dev/null
+++ b/src/librustdoc/theme.rs
@@ -0,0 +1,258 @@
+// Copyright 2012-2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::collections::HashSet;
+use std::fs::File;
+use std::hash::{Hash, Hasher};
+use std::io::Read;
+use std::path::Path;
+
+macro_rules! try_false {
+    ($e:expr) => ({
+        match $e {
+            Ok(c) => c,
+            Err(e) => {
+                eprintln!("rustdoc: got an error: {}", e);
+                return false;
+            }
+        }
+    })
+}
+
+#[derive(Debug, Clone, Eq)]
+pub struct CssPath {
+    pub name: String,
+    pub children: HashSet<CssPath>,
+}
+
+// This PartialEq implementation IS NOT COMMUTATIVE!!!
+//
+// The order is very important: the second object must have all first's rules.
+// However, the first doesn't require to have all second's rules.
+impl PartialEq for CssPath {
+    fn eq(&self, other: &CssPath) -> bool {
+        if self.name != other.name {
+            false
+        } else {
+            for child in &self.children {
+                if !other.children.iter().any(|c| child == c) {
+                    return false;
+                }
+            }
+            true
+        }
+    }
+}
+
+impl Hash for CssPath {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.name.hash(state);
+        for x in &self.children {
+            x.hash(state);
+        }
+    }
+}
+
+impl CssPath {
+    fn new(name: String) -> CssPath {
+        CssPath {
+            name,
+            children: HashSet::new(),
+        }
+    }
+}
+
+/// All variants contain the position they occur.
+#[derive(Debug, Clone, Copy)]
+enum Events {
+    StartLineComment(usize),
+    StartComment(usize),
+    EndComment(usize),
+    InBlock(usize),
+    OutBlock(usize),
+}
+
+impl Events {
+    fn get_pos(&self) -> usize {
+        match *self {
+            Events::StartLineComment(p) |
+            Events::StartComment(p) |
+            Events::EndComment(p) |
+            Events::InBlock(p) |
+            Events::OutBlock(p) => p,
+        }
+    }
+
+    fn is_comment(&self) -> bool {
+        match *self {
+            Events::StartLineComment(_) |
+            Events::StartComment(_) |
+            Events::EndComment(_) => true,
+            _ => false,
+        }
+    }
+}
+
+fn previous_is_line_comment(events: &[Events]) -> bool {
+    if let Some(&Events::StartLineComment(_)) = events.last() {
+        true
+    } else {
+        false
+    }
+}
+
+fn is_line_comment(pos: usize, v: &[u8], events: &[Events]) -> bool {
+    if let Some(&Events::StartComment(_)) = events.last() {
+        return false;
+    }
+    pos + 1 < v.len() && v[pos + 1] == b'/'
+}
+
+fn load_css_events(v: &[u8]) -> Vec<Events> {
+    let mut pos = 0;
+    let mut events = Vec::with_capacity(100);
+
+    while pos < v.len() - 1 {
+        match v[pos] {
+            b'/' if pos + 1 < v.len() && v[pos + 1] == b'*' => {
+                events.push(Events::StartComment(pos));
+                pos += 1;
+            }
+            b'/' if is_line_comment(pos, v, &events) => {
+                events.push(Events::StartLineComment(pos));
+                pos += 1;
+            }
+            b'\n' if previous_is_line_comment(&events) => {
+                events.push(Events::EndComment(pos));
+            }
+            b'*' if pos + 1 < v.len() && v[pos + 1] == b'/' => {
+                events.push(Events::EndComment(pos + 2));
+                pos += 1;
+            }
+            b'{' if !previous_is_line_comment(&events) => {
+                if let Some(&Events::StartComment(_)) = events.last() {
+                    pos += 1;
+                    continue
+                }
+                events.push(Events::InBlock(pos + 1));
+            }
+            b'}' if !previous_is_line_comment(&events) => {
+                if let Some(&Events::StartComment(_)) = events.last() {
+                    pos += 1;
+                    continue
+                }
+                events.push(Events::OutBlock(pos + 1));
+            }
+            _ => {}
+        }
+        pos += 1;
+    }
+    events
+}
+
+fn get_useful_next(events: &[Events], pos: &mut usize) -> Option<Events> {
+    while *pos < events.len() {
+        if !events[*pos].is_comment() {
+            return Some(events[*pos]);
+        }
+        *pos += 1;
+    }
+    None
+}
+
+fn inner(v: &[u8], events: &[Events], pos: &mut usize) -> HashSet<CssPath> {
+    let mut pathes = Vec::with_capacity(50);
+
+    while *pos < events.len() {
+        if let Some(Events::OutBlock(_)) = get_useful_next(events, pos) {
+            println!("00 => {:?}", events[*pos]);
+            *pos += 1;
+            break
+        }
+        println!("a => {:?}", events[*pos]);
+        if let Some(Events::InBlock(start_pos)) = get_useful_next(events, pos) {
+            println!("aa => {:?}", events[*pos]);
+            pathes.push(CssPath::new(::std::str::from_utf8(if *pos > 0 {
+                &v[events[*pos - 1].get_pos()..start_pos - 1]
+            } else {
+                &v[..start_pos]
+            }).unwrap_or("").trim().to_owned()));
+            *pos += 1;
+        }
+        println!("b => {:?}", events[*pos]);
+        while let Some(Events::InBlock(_)) = get_useful_next(events, pos) {
+            println!("bb => {:?}", events[*pos]);
+            if let Some(ref mut path) = pathes.last_mut() {
+                for entry in inner(v, events, pos).iter() {
+                    path.children.insert(entry.clone());
+                }
+            }
+        }
+        if *pos < events.len() {
+            println!("c => {:?}", events[*pos]);
+        }
+        if let Some(Events::OutBlock(_)) = get_useful_next(events, pos) {
+            *pos += 1;
+        }
+    }
+    pathes.iter().cloned().collect()
+}
+
+pub fn load_css_pathes(v: &[u8]) -> CssPath {
+    let events = load_css_events(v);
+    let mut pos = 0;
+
+    println!("\n======> {:?}", events);
+    let mut parent = CssPath::new("parent".to_owned());
+    parent.children = inner(v, &events, &mut pos);
+    parent
+}
+
+pub fn test_theme_against<P: AsRef<Path>>(f: &P, against: &CssPath) -> bool {
+    let mut file = try_false!(File::open(f));
+    let mut data = Vec::with_capacity(1000);
+
+    try_false!(file.read_to_end(&mut data));
+    let pathes = load_css_pathes(&data);
+    println!("========= {:?}", pathes);
+    println!("========= {:?}", against);
+    pathes == *against
+}
+
+#[test]
+fn test_comments_in_rules() {
+    let text = r#"
+rule a {}
+
+rule b, c
+// a line comment
+{}
+
+rule d
+// another line comment
+e {}
+
+rule f/* a multine
+
+comment*/{}
+
+rule g/* another multine
+
+comment*/h
+
+i {}
+
+rule j/*commeeeeent
+
+you like things like "{}" in there? :)
+*/
+end {}
+"#;
+}
\ No newline at end of file