web: add support of internatialization (#11) (#16)

README.md

@@ -166,13 +166,96 @@ After that you should set the directory path to the environment variable `WERTHE
 
 ### Custom login page
 
+A login page's template must be a Go template. The template has access to data conforming the next JSON-schema:
+
+```yaml
+type: object
+properties:
+  - WebBasePath:
+      description: The base path of the login page
+      type: string
+  - LangPrefs:
+      description: The user language preferences (the parsed value of the header Accept-Language)
+      type: array
+      items:
+        type: object
+        properties:
+          - Lang:
+              description: The language canonical name.
+              type: string
+          - Weight:
+              description: The language weight.
+              type: number
+        required:
+          - Lang
+          - Weight
+  - Data:
+      type: object
+      properties:
+        - CSRFToken:
+            description: A CSRF token.
+            type: string
+        - Challenge:
+            description: A login challenge ID.
+            type: string
+        - LoginURL:
+            description: An endpoint that finishes the login process.
+            type: string
+        - IsInvalidCredentials:
+            description: Specifies that a user types an invalid username or password.
+            type: boolean
+        - IsInternalError:
+            description: Specifies that an internal server error happens when finishing the login process.
+            type: boolean
+      required:
+        - CSRFToken
+        - Challenge
+        - LoginURL
+        - IsInvalidCredentials
+        - IsInternalError
+required:
+  - WebBasePath
+  - LangPrefs
+  - Data
+```
+
+When a login page's template contains static resources (like styles, scripts, and images)
+they must be placed in a subdirectory called `static`.
+
+For a full example of a login page's template see [source code](internal/web/templates).
+
+### Custom login page (old format)
+
+*The old template format is also supported but it will be removed in the future major release.*
+
 A login page's template should contains blocks `title`, `style`, `script`, `content`.
-Each block has access to data that is an object with the next properties:
+Each block has access to data conforming the next JSON-schema:
-- `CSRFToken` (string) - a CSRF token;
+
-- `Challenge` (string) - a login challenge ID;
+```yaml
-- `LoginURL` (string) - an endpoint that finishes the login process;
+type: object
-- `IsInvalidCredentials` (bool) - specifies that a user types an invalid username or password;
+properties:
-- `IsInternalError` (bool) specifies that an internal server error happens when finishing the login process.
+  - CSRFToken:
+    description: A CSRF token.
+    type: string
+  - Challenge:
+    description: A login challenge ID.
+    type: string
+  - LoginURL:
+    description: An endpoint that finishes the login process.
+    type: string
+  - IsInvalidCredentials:
+    description: Specifies that a user types an invalid username or password.
+    type: boolean
+  - IsInternalError:
+    description: Specifies that an internal server error happens when finishing the login process.
+    type: boolean
+required:
+  - CSRFToken
+  - Challenge
+  - LoginURL
+  - IsInvalidCredentials
+  - IsInternalError
+```
 
 When a login page's template contains static resources (like styles, scripts, and images)
 they must be placed in a subdirectory called `static`.

go.mod

@@ -17,7 +17,7 @@ require (
 	github.com/sergi/go-diff v1.0.0 // indirect
 	github.com/spaolacci/murmur3 v0.0.0-20180118202830-f09979ecbc72 // indirect
 	go.uber.org/zap v1.10.0
-	gopkg.in/asn1-ber.v1 v1.0.0-20170511165959-379148ca0225 // indirect
+	golang.org/x/text v0.3.2
 )
 
 go 1.13

go.sum

@@ -57,5 +57,6 @@ golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-gopkg.in/asn1-ber.v1 v1.0.0-20170511165959-379148ca0225 h1:JBwmEvLfCqgPcIq8MjVMQxsF3LVL4XG/HH0qiG0+IFY=
+golang.org/x/text v0.3.2 h1:tW2bmiBqwgJj/UpqtC8EpXEZVYOwU0yG4iWbprSVAcs=
-gopkg.in/asn1-ber.v1 v1.0.0-20170511165959-379148ca0225/go.mod h1:cuepJuh7vyXfUyUwEgHQXw849cJrilpS5NeIjOWESAw=
+golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=

internal/identp/identp.go

@@ -52,7 +52,7 @@ type oidcClaimsFinder interface {
 
 // TemplateRenderer renders a template with data and writes it to a http.ResponseWriter.
 type TemplateRenderer interface {
-	RenderTemplate(w http.ResponseWriter, name string, data interface{}) error
+	RenderTemplate(w http.ResponseWriter, r *http.Request, name string, data interface{}) error
 }
 
 // LoginTmplData is a data that is needed for rendering the login page.
@@ -147,7 +147,7 @@ func newLoginStartHandler(rproc oa2LoginReqProcessor, tmplRenderer TemplateRende
 			Challenge: challenge,
 			LoginURL:  strings.TrimPrefix(r.URL.String(), "/"),
 		}
-		if err := tmplRenderer.RenderTemplate(w, loginTmplName, data); err != nil {
+		if err := tmplRenderer.RenderTemplate(w, r, loginTmplName, data); err != nil {
 			log.Infow("Failed to render a login page template", zap.Error(err))
 			http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 			return
@@ -180,7 +180,7 @@ func newLoginEndHandler(ra oa2LoginReqAcceptor, auther authenticator, tmplRender
 			data.IsInternalError = true
 			log.Infow("Failed to authenticate a login request via the OAuth2 provider",
 				zap.Error(err), "challenge", challenge, "username", username)
-			if err = tmplRenderer.RenderTemplate(w, loginTmplName, data); err != nil {
+			if err = tmplRenderer.RenderTemplate(w, r, loginTmplName, data); err != nil {
 				log.Infow("Failed to render a login page template", zap.Error(err))
 				http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 			}
@@ -188,7 +188,7 @@ func newLoginEndHandler(ra oa2LoginReqAcceptor, auther authenticator, tmplRender
 		case !ok:
 			data.IsInvalidCredentials = true
 			log.Debugw("Invalid credentials", zap.Error(err), "challenge", challenge, "username", username)
-			if err = tmplRenderer.RenderTemplate(w, loginTmplName, data); err != nil {
+			if err = tmplRenderer.RenderTemplate(w, r, loginTmplName, data); err != nil {
 				log.Infow("Failed to render a login page template", zap.Error(err))
 				http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 			}
@@ -201,7 +201,7 @@ func newLoginEndHandler(ra oa2LoginReqAcceptor, auther authenticator, tmplRender
 		if err != nil {
 			data.IsInternalError = true
 			log.Infow("Failed to accept a login request via the OAuth2 provider", zap.Error(err))
-			if err := tmplRenderer.RenderTemplate(w, loginTmplName, data); err != nil {
+			if err := tmplRenderer.RenderTemplate(w, r, loginTmplName, data); err != nil {
 				log.Infow("Failed to render a login page template", zap.Error(err))
 				http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 			}

internal/identp/identp_test.go

@@ -98,7 +98,7 @@ func TestHandleLoginStart(t *testing.T) {
 			rr := httptest.NewRecorder()
 
 			tmplRenderer := &testTemplateRenderer{
-				renderTmplFunc: func(w http.ResponseWriter, name string, data interface{}) error {
+				renderTmplFunc: func(w http.ResponseWriter, r *http.Request, name string, data interface{}) error {
 					if name != "login.tmpl" {
 						t.Fatalf("wrong template name: got %q; want \"login.tmpl\"", name)
 					}
@@ -264,7 +264,7 @@ func TestHandleLoginEnd(t *testing.T) {
 			rr := httptest.NewRecorder()
 
 			tmplRenderer := &testTemplateRenderer{
-				renderTmplFunc: func(w http.ResponseWriter, name string, data interface{}) error {
+				renderTmplFunc: func(w http.ResponseWriter, r *http.Request, name string, data interface{}) error {
 					if name != "login.tmpl" {
 						t.Fatalf("wrong template name: got %q; want \"login.tmpl\"", name)
 					}
@@ -327,11 +327,11 @@ func TestHandleLoginEnd(t *testing.T) {
 }
 
 type testTemplateRenderer struct {
-	renderTmplFunc func(w http.ResponseWriter, name string, data interface{}) error
+	renderTmplFunc func(w http.ResponseWriter, r *http.Request, name string, data interface{}) error
 }
 
-func (tl *testTemplateRenderer) RenderTemplate(w http.ResponseWriter, name string, data interface{}) error {
+func (tl *testTemplateRenderer) RenderTemplate(w http.ResponseWriter, r *http.Request, name string, data interface{}) error {
-	return tl.renderTmplFunc(w, name, data)
+	return tl.renderTmplFunc(w, r, name, data)
 }
 
 type testAuthenticator struct {

internal/web/templates/login.tmpl

@@ -1,30 +1,26 @@
-{{ define "title" }}
+<!DOCTYPE html>
-	Login Provider Werther
+<html lang="{{ (index .LangPrefs 0).Lang }}">
-{{ end }}
+	<head>
-
+		<meta name="viewport" content="width=device-width, initial-scale=1">
-{{ define "style" }}
+		<title>Login Provider Werther</title>
+		<base href="{{ .WebBasePath }}">
 		<link rel="stylesheet" href="static/style.css">
-{{ end }}
+	</head>
-
+	<body>
-{{ define "js" }}
-	<script type="text/javascript" src="static/script.js"></script>
-{{ end }}
-
-{{ define "content" }}
 		<div class="login-page">
 			<div class="form">
 				<p class="message">
-			{{ if .IsInvalidCredentials }}
+				{{ if .Data.IsInvalidCredentials }}
 					Invalid username or password
-			{{ else if .IsInternalError }}
+				{{ else if .Data.IsInternalError }}
 					Internal server error
 				{{ else }}
 					&nbsp;
 				{{ end }}
 				</p>
-			<form class="login-form" action="{{ .LoginURL }}" method="POST">
+				<form class="login-form" action="{{ .Data.LoginURL }}" method="POST">
-				<input type="hidden" name="csrf_token" value={{ .CSRFToken }}>
+					<input type="hidden" name="csrf_token" value="{{ .Data.CSRFToken }}">
-				<input type="hidden" name="login_challenge" value={{ .Challenge }}>
+					<input type="hidden" name="login_challenge" value="{{ .Data.Challenge }}">
 
 					<input type="text" placeholder="username" name="username"/>
 					<input type="password" placeholder="password" name="password"/>
@@ -42,4 +38,6 @@
 				</form>
 			</div>
 		</div>
-{{ end }}
+		<script type="text/javascript" src="static/script.js"></script>
+	</body>
+</html>

internal/web/testdata/TestHTMLRenderer/external_template_happy_path/golden.file

@@ -1,32 +1,10 @@
 external template
 WebBasePath: testBasePath;
 
-Title:
+Langs:
-
+ru-RU;q=1,ru;q=0.9,en-US;q=0.8,en;q=0.7,
-CSRFToken: testCSRFToken;
-Challenge: testChalenge;
-LoginURL: testLoginURL;
-IsInvalidCredentials: true;
-IsInternalError: true;
-
-Style:
-
-CSRFToken: testCSRFToken;
-Challenge: testChalenge;
-LoginURL: testLoginURL;
-IsInvalidCredentials: true;
-IsInternalError: true;
-
-Js:
-
-CSRFToken: testCSRFToken;
-Challenge: testChalenge;
-LoginURL: testLoginURL;
-IsInvalidCredentials: true;
-IsInternalError: true;
-
-Content:
 
+Data:
 CSRFToken: testCSRFToken;
 Challenge: testChalenge;
 LoginURL: testLoginURL;

internal/web/testdata/TestHTMLRenderer/external_template_happy_path/login.tmpl

@@ -1,31 +1,13 @@
-{{- define "title" }}
+{{- define "main" }}external template
-CSRFToken: {{ .CSRFToken }};
+WebBasePath: {{ .WebBasePath }};
-Challenge: {{ .Challenge }};
-LoginURL: {{ .LoginURL }};
-IsInvalidCredentials: {{ .IsInvalidCredentials }};
-IsInternalError: {{ .IsInternalError }};
-{{- end }}
 
-{{- define "style" }}
+Langs:
-CSRFToken: {{ .CSRFToken }};
+{{ range .LangPrefs }}{{ .Lang }};q={{ .Weight }},{{ end }}
-Challenge: {{ .Challenge }};
-LoginURL: {{ .LoginURL }};
-IsInvalidCredentials: {{ .IsInvalidCredentials }};
-IsInternalError: {{ .IsInternalError }};
-{{- end }}
 
-{{- define "js" }}
+Data:
-CSRFToken: {{ .CSRFToken }};
+CSRFToken: {{ .Data.CSRFToken }};
-Challenge: {{ .Challenge }};
+Challenge: {{ .Data.Challenge }};
-LoginURL: {{ .LoginURL }};
+LoginURL: {{ .Data.LoginURL }};
-IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInvalidCredentials: {{ .Data.IsInvalidCredentials }};
-IsInternalError: {{ .IsInternalError }};
+IsInternalError: {{ .Data.IsInternalError }};
-{{- end }}
-
-{{- define "content" }}
-CSRFToken: {{ .CSRFToken }};
-Challenge: {{ .Challenge }};
-LoginURL: {{ .LoginURL }};
-IsInvalidCredentials: {{ .IsInvalidCredentials }};
-IsInternalError: {{ .IsInternalError }};
 {{- end }}

internal/web/testdata/TestHTMLRenderer/internal_template_happy_path/golden.file

@@ -1,32 +1,10 @@
 internal template
 WebBasePath: testBasePath;
 
-Title:
+Langs:
-
+ru-RU;q=1,ru;q=0.9,en-US;q=0.8,en;q=0.7,
-CSRFToken: testCSRFToken;
-Challenge: testChalenge;
-LoginURL: testLoginURL;
-IsInvalidCredentials: true;
-IsInternalError: true;
-
-Style:
-
-CSRFToken: testCSRFToken;
-Challenge: testChalenge;
-LoginURL: testLoginURL;
-IsInvalidCredentials: true;
-IsInternalError: true;
-
-Js:
-
-CSRFToken: testCSRFToken;
-Challenge: testChalenge;
-LoginURL: testLoginURL;
-IsInvalidCredentials: true;
-IsInternalError: true;
-
-Content:
 
+Data:
 CSRFToken: testCSRFToken;
 Challenge: testChalenge;
 LoginURL: testLoginURL;

internal/web/testdata/TestHTMLRenderer/internal_template_happy_path/login.tmpl

@@ -1,31 +1,13 @@
-{{- define "title" }}
+{{- define "main" }}internal template
-CSRFToken: {{ .CSRFToken }};
+WebBasePath: {{ .WebBasePath }};
-Challenge: {{ .Challenge }};
-LoginURL: {{ .LoginURL }};
-IsInvalidCredentials: {{ .IsInvalidCredentials }};
-IsInternalError: {{ .IsInternalError }};
-{{- end }}
 
-{{- define "style" }}
+Langs:
-CSRFToken: {{ .CSRFToken }};
+{{ range .LangPrefs }}{{ .Lang }};q={{ .Weight }},{{ end }}
-Challenge: {{ .Challenge }};
-LoginURL: {{ .LoginURL }};
-IsInvalidCredentials: {{ .IsInvalidCredentials }};
-IsInternalError: {{ .IsInternalError }};
-{{- end }}
 
-{{- define "js" }}
+Data:
-CSRFToken: {{ .CSRFToken }};
+CSRFToken: {{ .Data.CSRFToken }};
-Challenge: {{ .Challenge }};
+Challenge: {{ .Data.Challenge }};
-LoginURL: {{ .LoginURL }};
+LoginURL: {{ .Data.LoginURL }};
-IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInvalidCredentials: {{ .Data.IsInvalidCredentials }};
-IsInternalError: {{ .IsInternalError }};
+IsInternalError: {{ .Data.IsInternalError }};
-{{- end }}
-
-{{- define "content" }}
-CSRFToken: {{ .CSRFToken }};
-Challenge: {{ .Challenge }};
-LoginURL: {{ .LoginURL }};
-IsInvalidCredentials: {{ .IsInvalidCredentials }};
-IsInternalError: {{ .IsInternalError }};
 {{- end }}

internal/web/testdata/TestHTMLRenderer/old_style_external_template_happy_path/golden.file

@@ -0,0 +1,37 @@
+external template
+WebBasePath: testBasePath;
+
+Langs:
+ru-RU;q=1,ru;q=0.9,en-US;q=0.8,en;q=0.7,
+
+Title:
+
+CSRFToken: testCSRFToken;
+Challenge: testChalenge;
+LoginURL: testLoginURL;
+IsInvalidCredentials: true;
+IsInternalError: true;
+
+Style:
+
+CSRFToken: testCSRFToken;
+Challenge: testChalenge;
+LoginURL: testLoginURL;
+IsInvalidCredentials: true;
+IsInternalError: true;
+
+Js:
+
+CSRFToken: testCSRFToken;
+Challenge: testChalenge;
+LoginURL: testLoginURL;
+IsInvalidCredentials: true;
+IsInternalError: true;
+
+Content:
+
+CSRFToken: testCSRFToken;
+Challenge: testChalenge;
+LoginURL: testLoginURL;
+IsInvalidCredentials: true;
+IsInternalError: true;

internal/web/testdata/TestHTMLRenderer/old_style_external_template_happy_path/login.tmpl

@@ -0,0 +1,31 @@
+{{- define "title" }}
+CSRFToken: {{ .CSRFToken }};
+Challenge: {{ .Challenge }};
+LoginURL: {{ .LoginURL }};
+IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInternalError: {{ .IsInternalError }};
+{{- end }}
+
+{{- define "style" }}
+CSRFToken: {{ .CSRFToken }};
+Challenge: {{ .Challenge }};
+LoginURL: {{ .LoginURL }};
+IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInternalError: {{ .IsInternalError }};
+{{- end }}
+
+{{- define "js" }}
+CSRFToken: {{ .CSRFToken }};
+Challenge: {{ .Challenge }};
+LoginURL: {{ .LoginURL }};
+IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInternalError: {{ .IsInternalError }};
+{{- end }}
+
+{{- define "content" }}
+CSRFToken: {{ .CSRFToken }};
+Challenge: {{ .Challenge }};
+LoginURL: {{ .LoginURL }};
+IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInternalError: {{ .IsInternalError }};
+{{- end }}

internal/web/testdata/TestHTMLRenderer/old_style_external_template_happy_path/main.tmpl

@@ -1,6 +1,9 @@
 {{- define "main" }}external template
 WebBasePath: {{ .WebBasePath }};
 
+Langs:
+{{ range .LangPrefs }}{{ .Lang }};q={{ .Weight }},{{ end }}
+
 Title:
 {{ block "title" .Data }}{{ end }}
 

internal/web/testdata/TestHTMLRenderer/old_style_internal_template_happy_path/golden.file

@@ -0,0 +1,37 @@
+internal template
+WebBasePath: testBasePath;
+
+Langs:
+ru-RU;q=1,ru;q=0.9,en-US;q=0.8,en;q=0.7,
+
+Title:
+
+CSRFToken: testCSRFToken;
+Challenge: testChalenge;
+LoginURL: testLoginURL;
+IsInvalidCredentials: true;
+IsInternalError: true;
+
+Style:
+
+CSRFToken: testCSRFToken;
+Challenge: testChalenge;
+LoginURL: testLoginURL;
+IsInvalidCredentials: true;
+IsInternalError: true;
+
+Js:
+
+CSRFToken: testCSRFToken;
+Challenge: testChalenge;
+LoginURL: testLoginURL;
+IsInvalidCredentials: true;
+IsInternalError: true;
+
+Content:
+
+CSRFToken: testCSRFToken;
+Challenge: testChalenge;
+LoginURL: testLoginURL;
+IsInvalidCredentials: true;
+IsInternalError: true;

internal/web/testdata/TestHTMLRenderer/old_style_internal_template_happy_path/login.tmpl

@@ -0,0 +1,31 @@
+{{- define "title" }}
+CSRFToken: {{ .CSRFToken }};
+Challenge: {{ .Challenge }};
+LoginURL: {{ .LoginURL }};
+IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInternalError: {{ .IsInternalError }};
+{{- end }}
+
+{{- define "style" }}
+CSRFToken: {{ .CSRFToken }};
+Challenge: {{ .Challenge }};
+LoginURL: {{ .LoginURL }};
+IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInternalError: {{ .IsInternalError }};
+{{- end }}
+
+{{- define "js" }}
+CSRFToken: {{ .CSRFToken }};
+Challenge: {{ .Challenge }};
+LoginURL: {{ .LoginURL }};
+IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInternalError: {{ .IsInternalError }};
+{{- end }}
+
+{{- define "content" }}
+CSRFToken: {{ .CSRFToken }};
+Challenge: {{ .Challenge }};
+LoginURL: {{ .LoginURL }};
+IsInvalidCredentials: {{ .IsInvalidCredentials }};
+IsInternalError: {{ .IsInternalError }};
+{{- end }}

internal/web/testdata/TestHTMLRenderer/old_style_internal_template_happy_path/main.tmpl

@@ -1,6 +1,9 @@
 {{- define "main" }}internal template
 WebBasePath: {{ .WebBasePath }};
 
+Langs:
+{{ range .LangPrefs }}{{ .Lang }};q={{ .Weight }},{{ end }}
+
 Title:
 {{ block "title" .Data }}{{ end }}
 

internal/web/web.go

@@ -22,6 +22,7 @@ import (
 	assetfs "github.com/elazarl/go-bindata-assetfs"
 	"github.com/i-core/routegroup"
 	"github.com/pkg/errors"
+	"golang.org/x/text/language"
 )
 
 // The file systems provide templates and their resources that are stored in the application's internal assets.
@@ -74,8 +75,14 @@ func NewHTMLRenderer(cnf Config) (*HTMLRenderer, error) {
 	return &HTMLRenderer{Config: cnf, mainTmpl: mainTmpl, fs: fs}, nil
 }
 
+type langPref struct {
+	Lang   string
+	Weight float32
+}
+
 // RenderTemplate renders a HTML page from a template with the specified name using the specified data.
-func (r *HTMLRenderer) RenderTemplate(w http.ResponseWriter, name string, data interface{}) error {
+func (r *HTMLRenderer) RenderTemplate(w http.ResponseWriter, req *http.Request, name string, data interface{}) error {
+	// Read and parse the requested template.
 	f, err := r.fs.Open(name)
 	if err != nil {
 		if v, ok := err.(*os.PathError); ok {
@@ -89,20 +96,56 @@ func (r *HTMLRenderer) RenderTemplate(w http.ResponseWriter, name string, data i
 	if err != nil {
 		return fmt.Errorf("failed to read template %q: %s", name, err)
 	}
-	t, err := r.mainTmpl.Clone()
+	root, err := template.New("main").Parse(string(b))
-	if err != nil {
-		return errors.Wrapf(err, "failed to clone the main template for template %q: %s", name, err)
-	}
-	t, err = t.Parse(string(b))
 	if err != nil {
 		return errors.Wrapf(err, "failed to parse template %q: %s", name, err)
 	}
 
+	// The old-style template of a web page showed itself as not flexible.
+	// It was changed with a new template that allows overriding the whole page.
+	// The old-style template left for backward compatibility
+	// and will be deprecated in the future major release.
+	if isOldStyleUserTemplate(root) {
+		var wrapper *template.Template
+		wrapper, err = r.mainTmpl.Clone()
+		if err != nil {
+			return errors.Wrapf(err, "failed to clone the main template for template %q: %s", name, err)
+		}
+		root, err = root.AddParseTree("main", wrapper.Tree)
+		if err != nil {
+			return errors.Wrapf(err, "failed to create the main template for template %q: %s", name, err)
+		}
+	}
+
+	// Prepare template data.
+	basePath := r.BasePath
+	if basePath == "" {
+		basePath = "/"
+	}
+
+	var langPrefs []langPref
+	if acceptLang := req.Header.Get(http.CanonicalHeaderKey("Accept-Language")); acceptLang != "" {
+		var tags []language.Tag
+		var weights []float32
+		tags, weights, err = language.ParseAcceptLanguage(acceptLang)
+		if err != nil {
+			return errors.Wrapf(err, "failed to parse the header \"Accept-Language\": %s", err)
+		}
+		for i, tag := range tags {
+			langPrefs = append(langPrefs, langPref{Lang: tag.String(), Weight: weights[i]})
+		}
+	} else {
+		langPrefs = []langPref{{Lang: "en", Weight: 1}}
+	}
+
+	tmplData := map[string]interface{}{"WebBasePath": basePath, "LangPrefs": langPrefs, "Data": data}
+
+	// Render the template.
 	var (
 		buf bytes.Buffer
 		bw  = bufio.NewWriter(&buf)
 	)
-	if err = t.Execute(bw, map[string]interface{}{"WebBasePath": r.BasePath, "Data": data}); err != nil {
+	if err = root.Execute(bw, tmplData); err != nil {
 		return err
 	}
 	if err = bw.Flush(); err != nil {
@@ -111,16 +154,38 @@ func (r *HTMLRenderer) RenderTemplate(w http.ResponseWriter, name string, data i
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	_, err = buf.WriteTo(w)
 	return err
+}
 
+// Returns true if a template is the old-style template.
+//
+// A template is considered as the old-style template
+// if it contains four blocks for customizing the page title,
+// styles, markup, and scripts.
+//
+// See https://github.com/i-core/werther/issues/11.
+func isOldStyleUserTemplate(root *template.Template) bool {
+	var tmpls []string
+	for _, tmpl := range root.Templates() {
+		tmpls = append(tmpls, tmpl.Name())
+	}
+	contains := func(arr []string, tgt string) bool {
+		for _, item := range arr {
+			if item == tgt {
+				return true
+			}
+		}
+		return false
+	}
+	return contains(tmpls, "title") && contains(tmpls, "style") && contains(tmpls, "js") && contains(tmpls, "content")
 }
 
 var mainT = `{{ define "main" }}
 <!DOCTYPE html>
-<html>
+<html lang="{{ (index .LangPrefs 0).Lang }}">
 	<head>
 		<meta name="viewport" content="width=device-width, initial-scale=1">
 		<title>{{ block "title" .Data }}{{ end }}</title>
-		<base href={{ .WebBasePath }}>
+		<base href="{{ .WebBasePath }}">
 		{{ block "style" .Data }}{{ end }}
 	</head>
 	<body>

internal/web/web_test.go

@@ -60,23 +60,64 @@ func TestHTMLRenderer(t *testing.T) {
 				"IsInternalError":      true,
 			},
 		},
+
+		{
+			name:    "old style internal template not found",
+			wantErr: fmt.Errorf(`the template "login.tmpl" does not exist`),
+		},
+		{
+			name:     "old style internal template happy path",
+			basePath: "testBasePath",
+			data: map[string]interface{}{
+				"CSRFToken":            "testCSRFToken",
+				"Challenge":            "testChalenge",
+				"LoginURL":             "testLoginURL",
+				"IsInvalidCredentials": true,
+				"IsInternalError":      true,
+			},
+		},
+		{
+			name:    "old style external template not found",
+			ext:     true,
+			wantErr: fmt.Errorf(`the template "login.tmpl" does not exist`),
+		},
+		{
+			name:     "old style external template happy path",
+			ext:      true,
+			basePath: "testBasePath",
+			data: map[string]interface{}{
+				"CSRFToken":            "testCSRFToken",
+				"Challenge":            "testChalenge",
+				"LoginURL":             "testLoginURL",
+				"IsInvalidCredentials": true,
+				"IsInternalError":      true,
+			},
+		},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			tstDir := path.Join("testdata", t.Name())
 
-			// Read the main template.
 			var originMainT = mainT
 			defer func() { mainT = originMainT }()
-			f, err := os.Open(path.Join(tstDir, "main.tmpl"))
+
-			if err != nil {
+			// Read the main template if it is exist.
+			fpath := path.Join(tstDir, "main.tmpl")
+			stat, err := os.Stat(fpath)
+			if err != nil && !os.IsNotExist(err) {
 				t.Fatalf("failed to open main template: %s", err)
 			}
-			fc, err := ioutil.ReadAll(f)
+			if stat != nil {
-			if err != nil {
+				var f *os.File
+				if f, err = os.Open(fpath); err != nil {
+					t.Fatalf("failed to open main template: %s", err)
+				}
+				var fc []byte
+				if fc, err = ioutil.ReadAll(f); err != nil {
 					t.Fatalf("failed to read main template: %s", err)
 				}
 				mainT = string(fc)
+			}
 
 			// Create the template renderer.
 			cnf := Config{BasePath: tc.basePath}
@@ -93,7 +134,9 @@ func TestHTMLRenderer(t *testing.T) {
 			}
 
 			rr := httptest.NewRecorder()
-			err = r.RenderTemplate(rr, "login.tmpl", tc.data)
+			req := httptest.NewRequest(http.MethodGet, "http://localhost", nil)
+			req.Header.Set(http.CanonicalHeaderKey("Accept-Language"), "ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7")
+			err = r.RenderTemplate(rr, req, "login.tmpl", tc.data)
 
 			if tc.wantErr != nil {
 				if err == nil {
@@ -107,11 +150,11 @@ func TestHTMLRenderer(t *testing.T) {
 			if err != nil {
 				t.Fatalf("\ngot error\n\t%s\nwant no errors", err)
 			}
-			f, err = os.Open(path.Join(tstDir, "golden.file"))
+			f, err := os.Open(path.Join(tstDir, "golden.file"))
 			if err != nil {
 				t.Fatalf("failed to open golden file: %s", err)
 			}
-			fc, err = ioutil.ReadAll(f)
+			fc, err := ioutil.ReadAll(f)
 			if err != nil {
 				t.Fatalf("failed to read golden file: %s", err)
 			}