diff --git a/Cargo.lock b/Cargo.lock
index 4ea0c33c..fcc69153 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -69,12 +69,6 @@ dependencies = [
  "winapi",
 ]
 
-[[package]]
-name = "autocfg"
-version = "0.1.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1d49d90015b3c36167a20fe2810c5cd875ad504b39cff3d4eae7977e6b7c1cb2"
-
 [[package]]
 name = "autocfg"
 version = "1.0.1"
@@ -138,7 +132,7 @@ version = "0.2.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cc38c385bfd7e444464011bb24820f40dd1c76bcdfa1b78611cb7c2e5cafab75"
 dependencies = [
- "rustc_version",
+ "rustc_version 0.2.3",
 ]
 
 [[package]]
@@ -165,13 +159,10 @@ dependencies = [
 ]
 
 [[package]]
-name = "cloudabi"
-version = "0.0.3"
+name = "convert_case"
+version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ddfc5b9aa5d4507acaf872de71051dfd0e309860e88966e1051e462a077aac4f"
-dependencies = [
- "bitflags",
-]
+checksum = "6245d59a3e82a7fc217c5828a6692dbc6dfb63a0c8c90495621f7b9d79704a0e"
 
 [[package]]
 name = "core-foundation"
@@ -289,18 +280,16 @@ dependencies = [
 
 [[package]]
 name = "cssparser"
-version = "0.25.9"
+version = "0.28.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fbe18ca4efb9ba3716c6da66cc3d7e673bf59fa576353011f48c4cfddbdd740e"
+checksum = "1db8599a9761b371751fbf13e076fa03c6e1a78f8c5288e6ab9467f10a2322c1"
 dependencies = [
- "autocfg 0.1.7",
  "cssparser-macros",
  "dtoa-short",
  "itoa",
  "matches",
  "phf",
  "proc-macro2",
- "procedural-masquerade",
  "quote",
  "smallvec",
  "syn",
@@ -308,13 +297,10 @@ dependencies = [
 
 [[package]]
 name = "cssparser-macros"
-version = "0.3.6"
+version = "0.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5bb1c84e87c717666564ec056105052331431803d606bd45529b28547b611eef"
+checksum = "dfae75de57f2b2e85e8768c3ea840fd159c8f33e2b6522c7835b7abac81be16e"
 dependencies = [
- "phf_codegen",
- "proc-macro2",
- "procedural-masquerade",
  "quote",
  "syn",
 ]
@@ -341,6 +327,19 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "derive_more"
+version = "0.99.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4fb810d30a7c1953f91334de7244731fc3f3c10d7fe163338a35b9f640960321"
+dependencies = [
+ "convert_case",
+ "proc-macro2",
+ "quote",
+ "rustc_version 0.4.0",
+ "syn",
+]
+
 [[package]]
 name = "digest"
 version = "0.9.0"
@@ -423,12 +422,6 @@ dependencies = [
  "percent-encoding",
 ]
 
-[[package]]
-name = "fuchsia-cprng"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a06f77d526c1a601b7c4cdd98f54b5eaabffc14d5f2f0296febdc7f357c6d3ba"
-
 [[package]]
 name = "futures"
 version = "0.3.15"
@@ -483,7 +476,7 @@ version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a4c40298486cdf52cc00cd6d6987892ba502c7656a16a4192a9992b1ccedd121"
 dependencies = [
- "autocfg 1.0.1",
+ "autocfg",
  "proc-macro-hack",
  "proc-macro2",
  "quote",
@@ -508,7 +501,7 @@ version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "feb5c238d27e2bf94ffdfd27b2c29e3df4a68c4193bb6427384259e2bf191967"
 dependencies = [
- "autocfg 1.0.1",
+ "autocfg",
  "futures-channel",
  "futures-core",
  "futures-io",
@@ -542,6 +535,17 @@ dependencies = [
  "version_check",
 ]
 
+[[package]]
+name = "getrandom"
+version = "0.1.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8fc3cb4d91f53b50155bdcfd23f6a4c39ae1969c2ae85982b135750cccaf5fce"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "wasi 0.9.0+wasi-snapshot-preview1",
+]
+
 [[package]]
 name = "getrandom"
 version = "0.2.2"
@@ -550,7 +554,7 @@ checksum = "c9495705279e7140bf035dde1f6e750c162df8b625267cd52cc44e0b156732c8"
 dependencies = [
  "cfg-if",
  "libc",
- "wasi",
+ "wasi 0.10.2+wasi-snapshot-preview1",
 ]
 
 [[package]]
@@ -681,7 +685,7 @@ version = "1.6.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "824845a0bf897a9042383849b02c1bc219c2383772efcd5c6f9766fa4b81aef3"
 dependencies = [
- "autocfg 1.0.1",
+ "autocfg",
  "hashbrown",
 ]
 
@@ -757,12 +761,6 @@ version = "0.1.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7ffc5c5338469d4d3ea17d269fa8ea3512ad247247c30bd2df69e68309ed0a08"
 
-[[package]]
-name = "maybe-uninit"
-version = "2.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "60302e4db3a61da70c0cb7991976248362f30319e88850c487b9b95bbf059e00"
-
 [[package]]
 name = "memchr"
 version = "2.4.0"
@@ -775,7 +773,7 @@ version = "0.6.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f83fb6581e8ed1f85fd45c116db8405483899489e38406156c25eb743554361d"
 dependencies = [
- "autocfg 1.0.1",
+ "autocfg",
 ]
 
 [[package]]
@@ -791,7 +789,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a92518e98c078586bc6c934028adcca4c92a53d6a958196de835170a01d84e4b"
 dependencies = [
  "adler",
- "autocfg 1.0.1",
+ "autocfg",
 ]
 
 [[package]]
@@ -855,7 +853,7 @@ version = "0.2.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9a64b1ec5cda2586e284722486d802acf1f7dbdc623e2bfc57e65ca1cd099290"
 dependencies = [
- "autocfg 1.0.1",
+ "autocfg",
 ]
 
 [[package]]
@@ -912,7 +910,7 @@ version = "0.9.63"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b6b0d6fb7d80f877617dfcb014e605e2b5ab2fb0afdf27935219bb6bd984cb98"
 dependencies = [
- "autocfg 1.0.1",
+ "autocfg",
  "cc",
  "libc",
  "pkg-config",
@@ -927,18 +925,20 @@ checksum = "d4fd5641d01c8f18a23da7b6fe29298ff4b55afcccdf78973b24cf3175fee32e"
 
 [[package]]
 name = "phf"
-version = "0.7.24"
+version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b3da44b85f8e8dfaec21adae67f95d93244b2ecf6ad2a692320598dcc8e6dd18"
+checksum = "3dfb61232e34fcb633f43d12c58f83c1df82962dcdfa565a4e866ffc17dafe12"
 dependencies = [
+ "phf_macros",
  "phf_shared",
+ "proc-macro-hack",
 ]
 
 [[package]]
 name = "phf_codegen"
-version = "0.7.24"
+version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b03e85129e324ad4166b06b2c7491ae27fe3ec353af72e72cd1654c7225d517e"
+checksum = "cbffee61585b0411840d3ece935cce9cb6321f01c45477d30066498cd5e1a815"
 dependencies = [
  "phf_generator",
  "phf_shared",
@@ -946,19 +946,33 @@ dependencies = [
 
 [[package]]
 name = "phf_generator"
-version = "0.7.24"
+version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "09364cc93c159b8b06b1f4dd8a4398984503483891b0c26b867cf431fb132662"
+checksum = "17367f0cc86f2d25802b2c26ee58a7b23faeccf78a396094c13dced0d0182526"
 dependencies = [
  "phf_shared",
- "rand 0.6.5",
+ "rand 0.7.3",
+]
+
+[[package]]
+name = "phf_macros"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7f6fde18ff429ffc8fe78e2bf7f8b7a5a5a6e2a8b58bc5a9ac69198bbda9189c"
+dependencies = [
+ "phf_generator",
+ "phf_shared",
+ "proc-macro-hack",
+ "proc-macro2",
+ "quote",
+ "syn",
 ]
 
 [[package]]
 name = "phf_shared"
-version = "0.7.24"
+version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "234f71a15de2288bcb7e3b6515828d22af7ec8598ee6d24c3b526fa0a80b67a0"
+checksum = "c00cf8b9eafe68dde5e9eaa2cef8ee84a9336a47d566ec55ca16589633b65af7"
 dependencies = [
  "siphasher",
 ]
@@ -1062,12 +1076,6 @@ dependencies = [
  "unicode-xid",
 ]
 
-[[package]]
-name = "procedural-masquerade"
-version = "0.1.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8f1383dff4092fe903ac180e391a8d4121cc48f08ccf850614b0290c6673b69d"
-
 [[package]]
 name = "psl"
 version = "2.0.27"
@@ -1094,21 +1102,16 @@ dependencies = [
 
 [[package]]
 name = "rand"
-version = "0.6.5"
+version = "0.7.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6d71dacdc3c88c1fde3885a3be3fbab9f35724e6ce99467f7d9c5026132184ca"
+checksum = "6a6b1679d49b24bbfe0c803429aa1874472f50d9b363131f0e89fc356b544d03"
 dependencies = [
- "autocfg 0.1.7",
+ "getrandom 0.1.16",
  "libc",
- "rand_chacha 0.1.1",
- "rand_core 0.4.2",
- "rand_hc 0.1.0",
- "rand_isaac",
- "rand_jitter",
- "rand_os",
+ "rand_chacha 0.2.2",
+ "rand_core 0.5.1",
+ "rand_hc 0.2.0",
  "rand_pcg",
- "rand_xorshift",
- "winapi",
 ]
 
 [[package]]
@@ -1125,12 +1128,12 @@ dependencies = [
 
 [[package]]
 name = "rand_chacha"
-version = "0.1.1"
+version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "556d3a1ca6600bfcbab7c7c91ccb085ac7fbbcd70e008a98742e7847f4f7bcef"
+checksum = "f4c8ed856279c9737206bf725bf36935d8666ead7aa69b52be55af369d193402"
 dependencies = [
- "autocfg 0.1.7",
- "rand_core 0.3.1",
+ "ppv-lite86",
+ "rand_core 0.5.1",
 ]
 
 [[package]]
@@ -1145,35 +1148,29 @@ dependencies = [
 
 [[package]]
 name = "rand_core"
-version = "0.3.1"
+version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7a6fdeb83b075e8266dcc8762c22776f6877a63111121f5f8c7411e5be7eed4b"
+checksum = "90bde5296fc891b0cef12a6d03ddccc162ce7b2aff54160af9338f8d40df6d19"
 dependencies = [
- "rand_core 0.4.2",
+ "getrandom 0.1.16",
 ]
 
-[[package]]
-name = "rand_core"
-version = "0.4.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9c33a3c44ca05fa6f1807d8e6743f3824e8509beca625669633be0acbdf509dc"
-
 [[package]]
 name = "rand_core"
 version = "0.6.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "34cf66eb183df1c5876e2dcf6b13d57340741e8dc255b48e40a26de954d06ae7"
 dependencies = [
- "getrandom",
+ "getrandom 0.2.2",
 ]
 
 [[package]]
 name = "rand_hc"
-version = "0.1.0"
+version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7b40677c7be09ae76218dc623efbf7b18e34bced3f38883af07bb75630a21bc4"
+checksum = "ca3129af7b92a17112d59ad498c6f81eaf463253766b90396d39ea7a39d6613c"
 dependencies = [
- "rand_core 0.3.1",
+ "rand_core 0.5.1",
 ]
 
 [[package]]
@@ -1185,57 +1182,13 @@ dependencies = [
  "rand_core 0.6.2",
 ]
 
-[[package]]
-name = "rand_isaac"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ded997c9d5f13925be2a6fd7e66bf1872597f759fd9dd93513dd7e92e5a5ee08"
-dependencies = [
- "rand_core 0.3.1",
-]
-
-[[package]]
-name = "rand_jitter"
-version = "0.1.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1166d5c91dc97b88d1decc3285bb0a99ed84b05cfd0bc2341bdf2d43fc41e39b"
-dependencies = [
- "libc",
- "rand_core 0.4.2",
- "winapi",
-]
-
-[[package]]
-name = "rand_os"
-version = "0.1.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7b75f676a1e053fc562eafbb47838d67c84801e38fc1ba459e8f180deabd5071"
-dependencies = [
- "cloudabi",
- "fuchsia-cprng",
- "libc",
- "rand_core 0.4.2",
- "rdrand",
- "winapi",
-]
-
 [[package]]
 name = "rand_pcg"
-version = "0.1.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "abf9b09b01790cfe0364f52bf32995ea3c39f4d2dd011eac241d2914146d0b44"
-dependencies = [
- "autocfg 0.1.7",
- "rand_core 0.4.2",
-]
-
-[[package]]
-name = "rand_xorshift"
-version = "0.1.1"
+version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cbf7e9e623549b0e21f6e97cf8ecf247c1a8fd2e8a992ae265314300b2455d5c"
+checksum = "16abd0c1b639e9eb4d7c50c0b8100b0d0f849be2349829c740fe8e6eb4816429"
 dependencies = [
- "rand_core 0.3.1",
+ "rand_core 0.5.1",
 ]
 
 [[package]]
@@ -1244,7 +1197,7 @@ version = "1.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8b0d8e0819fadc20c74ea8373106ead0600e3a67ef1fe8da56e39b9ae7275674"
 dependencies = [
- "autocfg 1.0.1",
+ "autocfg",
  "crossbeam-deque",
  "either",
  "rayon-core",
@@ -1263,15 +1216,6 @@ dependencies = [
  "num_cpus",
 ]
 
-[[package]]
-name = "rdrand"
-version = "0.4.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "678054eb77286b51581ba43620cc911abf02758c91f93f479767aed0f90458b2"
-dependencies = [
- "rand_core 0.3.1",
-]
-
 [[package]]
 name = "redox_syscall"
 version = "0.2.8"
@@ -1388,7 +1332,16 @@ version = "0.2.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "138e3e0acb6c9fb258b19b67cb8abd63c00679d2851805ea151465464fe9030a"
 dependencies = [
- "semver",
+ "semver 0.9.0",
+]
+
+[[package]]
+name = "rustc_version"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bfa0f585226d2e68097d4f95d113b15b83a82e819ab25717ec0590d9584ef366"
+dependencies = [
+ "semver 1.0.4",
 ]
 
 [[package]]
@@ -1453,21 +1406,20 @@ dependencies = [
 
 [[package]]
 name = "selectors"
-version = "0.21.0"
+version = "0.23.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1b86b100bede4f651059740afc3b6cb83458d7401cb7c1ad96d8a11e91742c86"
+checksum = "fdea87c686be721aab36607728047801ee21561bfdbd6bf0da7ace2536d5879f"
 dependencies = [
  "bitflags",
  "cssparser",
+ "derive_more",
  "fxhash",
  "log",
- "matches",
  "phf",
  "phf_codegen",
  "precomputed-hash",
  "servo_arc",
  "smallvec",
- "thin-slice",
 ]
 
 [[package]]
@@ -1479,6 +1431,12 @@ dependencies = [
  "semver-parser",
 ]
 
+[[package]]
+name = "semver"
+version = "1.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "568a8e6258aa33c13358f81fd834adb854c6f7c9468520910a9b1e8fac068012"
+
 [[package]]
 name = "semver-parser"
 version = "0.7.0"
@@ -1563,9 +1521,9 @@ dependencies = [
 
 [[package]]
 name = "siphasher"
-version = "0.2.3"
+version = "0.3.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0b8de496cf83d4ed58b6be86c3a275b8602f6ffe98d3024a869e124147a9a3ac"
+checksum = "533494a8f9b724d33625ab53c6c4800f7cc445895924a8ef649222dcb76e938b"
 
 [[package]]
 name = "slab"
@@ -1575,12 +1533,9 @@ checksum = "f173ac3d1a7e3b28003f40de0b5ce7fe2710f9b9dc3fc38664cebee46b3b6527"
 
 [[package]]
 name = "smallvec"
-version = "0.6.14"
+version = "1.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b97fcaeba89edba30f044a10c6a3cc39df9c3f17d7cd829dd1446cab35f890e0"
-dependencies = [
- "maybe-uninit",
-]
+checksum = "1ecab6c735a6bb4139c0caafd0cc3635748bbb3acf4550e8138122099251f309"
 
 [[package]]
 name = "socket2"
@@ -1632,12 +1587,6 @@ dependencies = [
  "unicode-width",
 ]
 
-[[package]]
-name = "thin-slice"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8eaa81235c7058867fa8c0e7314f33dcce9c215f535d1913822a2b3f5e289f3c"
-
 [[package]]
 name = "tinytemplate"
 version = "1.2.1"
@@ -1669,7 +1618,7 @@ version = "1.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "570c2eb13b3ab38208130eccd41be92520388791207fde783bda7c1e8ace28d4"
 dependencies = [
- "autocfg 1.0.1",
+ "autocfg",
  "bytes",
  "libc",
  "memchr",
@@ -1832,6 +1781,12 @@ dependencies = [
  "try-lock",
 ]
 
+[[package]]
+name = "wasi"
+version = "0.9.0+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cccddf32554fecc6acb585f82a32a72e28b48f8c4c1883ddfeeeaa96f7d8e519"
+
 [[package]]
 name = "wasi"
 version = "0.10.2+wasi-snapshot-preview1"
diff --git a/Cargo.toml b/Cargo.toml
index 1b3d8326..de9a0877 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -33,8 +33,8 @@ base64 = "0.13"
 rmp-serde-legacy = { package = "rmp-serde", version = "0.13.7" }    # rmp-serde 0.14.0 breaks deserialization by changing how enums are deserialized
 rmp-serde = "0.15"
 lifeguard = { version = "^ 0.6.1", optional = true }
-cssparser = { version = "0.25", optional = true }
-selectors = { version = "0.21", optional = true }
+cssparser = { version = "0.28", optional = true }
+selectors = { version = "0.23", optional = true }
 serde_json = { version = "1.0", optional = true }
 
 [dev-dependencies]
diff --git a/src/filters/cosmetic.rs b/src/filters/cosmetic.rs
index a8d0501e..01ccd4ea 100644
--- a/src/filters/cosmetic.rs
+++ b/src/filters/cosmetic.rs
@@ -423,18 +423,81 @@ mod css_validation {
 #[cfg(feature="css-validation")]
 mod css_validation {
     //! Methods for validating CSS selectors and style rules extracted from cosmetic filter rules.
-    use cssparser::ParserInput;
-    use cssparser::Parser;
-    use selectors::parser::Selector;
-
-    use std::fmt::{Display, Formatter, Error};
+    use cssparser::{CowRcStr, ParserInput, Parser, ParseError, SourceLocation};
     use core::fmt::{Write, Result as FmtResult};
 
+    /// Checks whether the given string represents a valid CSS selector.
+    ///
+    /// For the majority of filters, this works by trivial regex matching. More complex filters are
+    /// assembled into a mock stylesheet which is then parsed using `cssparser` and validated.
+    ///
+    /// This function does not attempt to validate specific types of pseudoclasses, since the list
+    /// of valid ones is large and constantly evolving. However, it should still catch any kind of
+    /// comment escaping that would allow injection of `url()` styles.
     pub fn is_valid_css_selector(selector: &str) -> bool {
-        let mut pi = ParserInput::new(selector);
+        use once_cell::sync::Lazy;
+        use regex::Regex;
+        static RE_SIMPLE_SELECTOR: Lazy<Regex> = Lazy::new(|| Regex::new(r"^[#.]?[A-Za-z_][\w-]*$").unwrap());
+
+        if RE_SIMPLE_SELECTOR.is_match(selector) {
+            return true;
+        }
+
+        // Use `mock-stylesheet-marker` where uBO uses `color: red` since we have control over the
+        // parsing logic within the block.
+        let mock_stylesheet = format!("{}{{mock-stylesheet-marker}}", selector);
+        let mut pi = ParserInput::new(&mock_stylesheet);
         let mut parser = Parser::new(&mut pi);
-        let r = Selector::parse(&SelectorParseImpl, &mut parser);
-        r.is_ok()
+        let mut rule_list_parser = cssparser::RuleListParser::new_for_stylesheet(&mut parser, QualifiedRuleParserImpl);
+
+        if let Some(first_rule) = rule_list_parser.next() {
+            match first_rule {
+                Ok(()) => (),
+                Err(_) => return false,
+            }
+        } else {
+            return false;
+        }
+
+        let is_last_rule = rule_list_parser.next().is_none();
+
+        is_last_rule
+    }
+
+    struct QualifiedRuleParserImpl;
+
+    impl<'i> cssparser::QualifiedRuleParser<'i> for QualifiedRuleParserImpl {
+        type Prelude = ();
+        type QualifiedRule = ();
+        type Error = ();
+
+        fn parse_prelude<'t>(&mut self, input: &mut Parser<'i, 't>) -> Result<Self::Prelude, ParseError<'i, Self::Error>> {
+            selectors::SelectorList::parse(&SelectorParseImpl, input)
+                .map(|_| ())
+                .map_err(|_| ParseError { kind: cssparser::ParseErrorKind::Custom(()), location: SourceLocation { line: 0, column: 0 } })
+        }
+
+        /// Check that the block is exactly equal to "mock-stylesheet-marker"
+        fn parse_block<'t>(&mut self, _prelude: Self::Prelude, _start: &cssparser::ParserState, input: &mut Parser<'i, 't>) -> Result<Self::QualifiedRule, ParseError<'i, Self::Error>> {
+            let err = Err(ParseError { kind: cssparser::ParseErrorKind::Custom(()), location: SourceLocation { line: 0, column: 0 } });
+            match input.next() {
+                Ok(cssparser::Token::Ident(i)) if i.as_ref() == "mock-stylesheet-marker" => (),
+                _ => return err,
+            }
+            if input.next().is_ok() {
+                return err;
+            }
+            Ok(())
+        }
+    }
+
+    /// Default implementations for `AtRuleParser` parsing methods return false. This is
+    /// acceptable; at-rules should not be valid in cosmetic rules.
+    impl cssparser::AtRuleParser<'_> for QualifiedRuleParserImpl {
+        type PreludeNoBlock = ();
+        type PreludeBlock = ();
+        type AtRule = ();
+        type Error = ();
     }
 
     pub fn is_valid_css_style(style: &str) -> bool {
@@ -455,6 +518,25 @@ mod css_validation {
     impl<'i> selectors::parser::Parser<'i> for SelectorParseImpl {
         type Impl = SelectorImpl;
         type Error = selectors::parser::SelectorParseErrorKind<'i>;
+
+        fn parse_slotted(&self) -> bool { true }
+        fn parse_part(&self) -> bool { true }
+        fn parse_is_and_where(&self) -> bool { true }
+        fn parse_host(&self) -> bool { true }
+        fn parse_non_ts_pseudo_class(&self, _location: SourceLocation, _name: CowRcStr<'i>) -> Result<<Self::Impl as selectors::parser::SelectorImpl>::NonTSPseudoClass, ParseError<'i, Self::Error>> {
+            Ok(NonTSPseudoClass)
+        }
+        fn parse_non_ts_functional_pseudo_class<'t>(&self, _name: CowRcStr<'i>, arguments: &mut Parser<'i, 't>) -> Result<<Self::Impl as selectors::parser::SelectorImpl>::NonTSPseudoClass, ParseError<'i, Self::Error>> {
+            while arguments.next().is_ok() {}
+            Ok(NonTSPseudoClass)
+        }
+        fn parse_pseudo_element(&self, _location: SourceLocation, _name: CowRcStr<'i>) -> Result<<Self::Impl as selectors::parser::SelectorImpl>::PseudoElement, ParseError<'i, Self::Error>> {
+            Ok(PseudoElement)
+        }
+        fn parse_functional_pseudo_element<'t>(&self, _name: CowRcStr<'i>, arguments: &mut Parser<'i, 't>) -> Result<<Self::Impl as selectors::parser::SelectorImpl>::PseudoElement, ParseError<'i, Self::Error>> {
+            while arguments.next().is_ok() {}
+            Ok(PseudoElement)
+        }
     }
 
     /// The `selectors` library requires an object that implements `SelectorImpl` to store data
@@ -468,23 +550,24 @@ mod css_validation {
         type ExtraMatchingData = ();
         type AttrValue = DummyValue;
         type Identifier = DummyValue;
-        type ClassName = DummyValue;
-        type LocalName = String;
-        type NamespaceUrl = String;
+        type LocalName = DummyValue;
+        type NamespaceUrl = DummyValue;
         type NamespacePrefix = DummyValue;
-        type BorrowedNamespaceUrl = String;
-        type BorrowedLocalName = String;
+        type BorrowedNamespaceUrl = DummyValue;
+        type BorrowedLocalName = DummyValue;
         type NonTSPseudoClass = NonTSPseudoClass;
         type PseudoElement = PseudoElement;
     }
 
-    /// For performance, individual fields of parsed selectors is discarded. Instead, they are
+    /// For performance, individual fields of parsed selectors are discarded. Instead, they are
     /// parsed into a `DummyValue` with no fields.
     #[derive(Debug, Clone, PartialEq, Eq, Default)]
     struct DummyValue;
 
-    impl Display for DummyValue {
-        fn fmt(&self, _: &mut Formatter) -> Result<(), Error> { Ok(()) }
+    impl cssparser::ToCss for DummyValue {
+        fn to_css<W>(&self, _dest: &mut W) -> core::fmt::Result {
+            Ok(())
+        }
     }
 
     impl<'a> From<&'a str> for DummyValue {
@@ -498,6 +581,7 @@ mod css_validation {
     impl selectors::parser::NonTSPseudoClass for NonTSPseudoClass {
         type Impl = SelectorImpl;
         fn is_active_or_hover(&self) -> bool { false }
+        fn is_user_action_state(&self) -> bool { false }
     }
 
     impl cssparser::ToCss for NonTSPseudoClass {
@@ -511,8 +595,6 @@ mod css_validation {
     impl selectors::parser::PseudoElement for PseudoElement {
         type Impl = SelectorImpl;
 
-        fn supports_pseudo_class(&self, _pseudo_class: &NonTSPseudoClass) -> bool { true }
-
         fn valid_after_slotted(&self) -> bool { true }
     }
 
@@ -523,12 +605,17 @@ mod css_validation {
     #[test]
     fn bad_selector_inputs() {
         assert!(!is_valid_css_selector(r#"rm -rf ./*"#));
-        assert!(!is_valid_css_selector(r#"javascript:alert("hacked")"#));
+        assert!(is_valid_css_selector(r#"javascript:alert("All pseudo-classes are valid")"#));
+        assert!(!is_valid_css_selector(r#"javascript:alert("But opening comments are still forbidden" /*)"#));
         assert!(!is_valid_css_selector(r#"This is not a CSS selector."#));
         assert!(!is_valid_css_selector(r#"./malware.sh"#));
         assert!(!is_valid_css_selector(r#"https://safesite.ru"#));
         assert!(!is_valid_css_selector(r#"(function(){var e=60;return String.fromCharCode(e.charCodeAt(0))})();"#));
         assert!(!is_valid_css_selector(r#"#!/usr/bin/sh"#));
+        assert!(!is_valid_css_selector(r#"input,input/*"#));
+        // Accept a closing comment within a string. It should still be impossible to create an
+        // opening comment to match it.
+        assert!(is_valid_css_selector(r#"input[x="*/{}*{background:url(https://hackvertor.co.uk/images/logo.gif)}"]"#));
     }
 }
 
diff --git a/src/lists.rs b/src/lists.rs
index 9de78cdb..49d952cf 100644
--- a/src/lists.rs
+++ b/src/lists.rs
@@ -497,10 +497,24 @@ mod tests {
     #[test]
     fn parse_filter_failed_fuzz_4() {
         // \\##+js(,\xdd\x8d
-        assert!(parse_filter(
+        let parsed = parse_filter(
             &String::from_utf8(vec![92, 35, 35, 43, 106, 115, 40, 44, 221, 141]).unwrap(),
             true,
             Default::default(),
-        ).is_ok());
+        );
+        #[cfg(feature = "css-validation")]
+        assert!(parsed.is_err());
+        #[cfg(not(feature = "css-validation"))]
+        assert!(parsed.is_ok());
+    }
+
+    #[test]
+    #[cfg(feature = "css-validation")]
+    fn parse_filter_opening_comment() {
+        assert!(parse_filter(
+            "##input,input/*",
+            true,
+            Default::default(),
+        ).is_err());
     }
 }