diff --git a/Makefile b/Makefile
index dcd042a..97e9541 100644
--- a/Makefile
+++ b/Makefile
@@ -6,7 +6,7 @@
 #   fmt-check:    Check if the source files are formated
 #   build:        Builds the code locally
 #   vet:          Vets the code
-#   lint:         Runs lint over the code (you do not need to fix everything)
+#   staticcheck:  Runs staticcheck over the code
 #   test:         Runs the tests
 #   cover:        Gives you the URL to a nice test coverage report
 #
@@ -33,8 +33,8 @@ build:
 vet:
 	@go vet
 
-lint:
-	@golint *.go
+staticcheck:
+	@staticcheck ./...
 
 test:
 	@go test -v ./...
@@ -42,7 +42,7 @@ test:
 cover: COVERAGE_FILE := coverage.out
 cover:
 	@go test -coverprofile=$(COVERAGE_FILE) && \
-	cover -html=$(COVERAGE_FILE) && rm $(COVERAGE_FILE)
+	go tool cover -html=$(COVERAGE_FILE) && rm $(COVERAGE_FILE)
 
 install:
 	@go install ./...
diff --git a/cmd/sanitise_html_email/main.go b/cmd/sanitise_html_email/main.go
index 9f1f91f..86c2810 100644
--- a/cmd/sanitise_html_email/main.go
+++ b/cmd/sanitise_html_email/main.go
@@ -2,7 +2,7 @@ package main
 
 import (
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log"
 	"os"
 	"regexp"
@@ -61,7 +61,7 @@ func main() {
 
 	// Read input from stdin so that this is a nice unix utility and can receive
 	// piped input
-	dirty, err := ioutil.ReadAll(os.Stdin)
+	dirty, err := io.ReadAll(os.Stdin)
 	if err != nil {
 		log.Fatal(err)
 	}
diff --git a/cmd/sanitise_ugc/main.go b/cmd/sanitise_ugc/main.go
index 5fb8483..13515cf 100644
--- a/cmd/sanitise_ugc/main.go
+++ b/cmd/sanitise_ugc/main.go
@@ -2,7 +2,7 @@ package main
 
 import (
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log"
 	"os"
 
@@ -22,7 +22,7 @@ func main() {
 
 	// Read input from stdin so that this is a nice unix utility and can receive
 	// piped input
-	dirty, err := ioutil.ReadAll(os.Stdin)
+	dirty, err := io.ReadAll(os.Stdin)
 	if err != nil {
 		log.Fatal(err)
 	}
diff --git a/css/handlers.go b/css/handlers.go
index e0429cf..f8b8b61 100644
--- a/css/handlers.go
+++ b/css/handlers.go
@@ -366,10 +366,11 @@ func in(value []string, arr []string) bool {
 
 func splitValues(value string) []string {
 	values := strings.Split(value, ",")
+	newValues := []string{}
 	for _, strippedValue := range values {
-		strippedValue = strings.ToLower(strings.TrimSpace(strippedValue))
+		newValues = append(newValues, strings.ToLower(strings.TrimSpace(strippedValue)))
 	}
-	return values
+	return newValues
 }
 
 func GetDefaultHandler(attr string) func(string) bool {
diff --git a/go.mod b/go.mod
index f004b9f..9712d70 100644
--- a/go.mod
+++ b/go.mod
@@ -1,12 +1,16 @@
 module github.com/microcosm-cc/bluemonday
 
-go 1.19
+go 1.21
 
 require (
 	github.com/aymerick/douceur v0.2.0
-	golang.org/x/net v0.14.0
+	golang.org/x/net v0.17.0
 )
 
-require github.com/gorilla/css v1.0.0 // indirect
+require (
+	github.com/PuerkitoBio/goquery v1.8.1 // indirect
+	github.com/andybalholm/cascadia v1.3.2 // indirect
+	github.com/gorilla/css v1.0.0 // indirect
+)
 
 retract [v1.0.0, v1.0.24] // Retract older versions as only latest is to be depended upon
diff --git a/go.sum b/go.sum
index 6c31332..1ae1358 100644
--- a/go.sum
+++ b/go.sum
@@ -1,6 +1,50 @@
+github.com/PuerkitoBio/goquery v1.8.1 h1:uQxhNlArOIdbrH1tr0UXwdVFgDcZDrZVdcpygAcwmWM=
+github.com/PuerkitoBio/goquery v1.8.1/go.mod h1:Q8ICL1kNUJ2sXGoAhPGUdYDJvgQgHzJsnnd3H7Ho5jQ=
+github.com/andybalholm/cascadia v1.3.1 h1:nhxRkql1kdYCc8Snf7D5/D3spOX+dBgjA6u8x004T2c=
+github.com/andybalholm/cascadia v1.3.1/go.mod h1:R4bJ1UQfqADjvDa4P6HZHLh/3OxWWEqc0Sk8XGwHqvA=
+github.com/andybalholm/cascadia v1.3.2 h1:3Xi6Dw5lHF15JtdcmAHD3i1+T8plmv7BQ/nsViSLyss=
+github.com/andybalholm/cascadia v1.3.2/go.mod h1:7gtRlve5FxPPgIgX36uWBX58OdBsSS6lUvCFb+h7KvU=
 github.com/aymerick/douceur v0.2.0 h1:Mv+mAeH1Q+n9Fr+oyamOlAkUNPWPlA8PPGR0QAaYuPk=
 github.com/aymerick/douceur v0.2.0/go.mod h1:wlT5vV2O3h55X9m7iVYN0TBM0NH/MmbLnd30/FjWUq4=
 github.com/gorilla/css v1.0.0 h1:BQqNyPTi50JCFMTw/b67hByjMVXZRwGha6wxVGkeihY=
 github.com/gorilla/css v1.0.0/go.mod h1:Dn721qIggHpt4+EFCcTLTU/vk5ySda2ReITrtgBl60c=
-golang.org/x/net v0.14.0 h1:BONx9s002vGdD9umnlX1Po8vOZmrgH34qlHcD1MfK14=
-golang.org/x/net v0.14.0/go.mod h1:PpSgVXXLK0OxS0F31C1/tv6XNguvCrnXIDrFMspZIUI=
+github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
+golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
+golang.org/x/mod v0.8.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
+golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
+golang.org/x/net v0.0.0-20210916014120-12bc252f5db8/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
+golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug+ECip1KBveYUHfp+8e9klMJ9c=
+golang.org/x/net v0.6.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
+golang.org/x/net v0.7.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
+golang.org/x/net v0.9.0/go.mod h1:d48xBJpPfHeWQsugry2m+kC02ZBRGRgulfHnEXEuWns=
+golang.org/x/net v0.17.0 h1:pVaXccu2ozPjCXewfr1S7xza/zcXTity9cCdXQYSjIM=
+golang.org/x/net v0.17.0/go.mod h1:NxSsAGuq816PNPmqtQdLE42eU2Fs7NoRIZrHJAlaCOE=
+golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.5.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.7.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
+golang.org/x/term v0.0.0-20210927222741-03fcf44c2211/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
+golang.org/x/term v0.5.0/go.mod h1:jMB1sMXY+tzblOD4FWmEbocvup2/aLOaQEp7JmGp78k=
+golang.org/x/term v0.7.0/go.mod h1:P32HKFT3hSsZrRxla30E9HqToFYAQPCMs/zFMBUFqPY=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
+golang.org/x/text v0.7.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
+golang.org/x/text v0.9.0/go.mod h1:e1OnstbJyHTd6l/uOt8jFFHp6TRDWZR/bV3emEE/zU8=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
+golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
+golang.org/x/tools v0.6.0/go.mod h1:Xwgl3UAJ/d3gWutnCtw505GrjyAbvKui8lOU390QaIU=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
diff --git a/sanitize.go b/sanitize.go
index 2792fb3..1f8d855 100644
--- a/sanitize.go
+++ b/sanitize.go
@@ -998,7 +998,7 @@ func linkable(elementName string) bool {
 // stringInSlice returns true if needle exists in haystack
 func stringInSlice(needle string, haystack []string) bool {
 	for _, straw := range haystack {
-		if strings.ToLower(straw) == strings.ToLower(needle) {
+		if strings.EqualFold(straw, needle) {
 			return true
 		}
 	}