diff --git a/nbconvert/exporters/html.py b/nbconvert/exporters/html.py
index 30c10b7f0..055e69915 100644
--- a/nbconvert/exporters/html.py
+++ b/nbconvert/exporters/html.py
@@ -12,8 +12,9 @@
 
 import jinja2
 import markupsafe
+from bs4 import BeautifulSoup
 from jupyter_core.paths import jupyter_path
-from traitlets import Bool, Unicode, default
+from traitlets import Bool, Unicode, default, validate
 from traitlets.config import Config
 
 if tuple(int(x) for x in jinja2.__version__.split(".")[:3]) < (3, 0, 0):
@@ -27,6 +28,7 @@
 from nbconvert.filters.highlight import Highlight2HTML
 from nbconvert.filters.markdown_mistune import IPythonRenderer, MarkdownWithMath
 from nbconvert.filters.widgetsdatatypefilter import WidgetsDataTypeFilter
+from nbconvert.utils.iso639_1 import iso639_1
 
 from .templateexporter import TemplateExporter
 
@@ -202,6 +204,20 @@ def default_config(self):
             c = c2
         return c
 
+    language_code = Unicode(
+        "en", help="Language code of the content, should be one of the ISO639-1"
+    ).tag(config=True)
+
+    @validate("language_code")
+    def _valid_language_code(self, proposal):
+        if self.language_code not in iso639_1:
+            self.log.warn(
+                f'"{self.language_code}" is not an ISO 639-1 language code. '
+                'It has been replaced by the default value "en".'
+            )
+            return proposal["trait"].default_value
+        return proposal["value"]
+
     @contextfilter
     def markdown2html(self, context, source):
         """Markdown to HTML filter respecting the anchor_link_text setting"""
@@ -240,7 +256,18 @@ def from_notebook_node(  # type:ignore
 
         self.register_filter("highlight_code", highlight_code)
         self.register_filter("filter_data_type", filter_data_type)
-        return super().from_notebook_node(nb, resources, **kw)
+        html, resources = super().from_notebook_node(nb, resources, **kw)
+        soup = BeautifulSoup(html, features="html.parser")
+        # Add image's alternative text
+        for elem in soup.select("img:not([alt])"):
+            elem.attrs["alt"] = "Image"
+        # Set input and output focusable
+        for elem in soup.select(".jp-Notebook div.jp-Cell-inputWrapper"):
+            elem.attrs["tabindex"] = "0"
+        for elem in soup.select(".jp-Notebook div.jp-OutputArea-output"):
+            elem.attrs["tabindex"] = "0"
+
+        return str(soup), resources
 
     def _init_resources(self, resources):  # noqa
         def resources_include_css(name):
@@ -318,4 +345,5 @@ def resources_include_url(name):
         resources["widget_renderer_url"] = self.widget_renderer_url
         resources["html_manager_semver_range"] = self.html_manager_semver_range
         resources["should_sanitize_html"] = self.sanitize_html
+        resources["language_code"] = self.language_code
         return resources
diff --git a/nbconvert/exporters/tests/test_html.py b/nbconvert/exporters/tests/test_html.py
index 6d5db7c3c..04c2a74f8 100644
--- a/nbconvert/exporters/tests/test_html.py
+++ b/nbconvert/exporters/tests/test_html.py
@@ -76,12 +76,12 @@ def test_png_metadata(self):
         (output, resources) = HTMLExporter(template_name="classic").from_filename(
             self._get_notebook(nb_name="pngmetadata.ipynb")
         )
-        check_for_png = re.compile(r'<img src="[^"]*?"([^>]*?)>')
+        check_for_png = re.compile(r'<img alt="Image"([^>]*?)>')
         result = check_for_png.search(output)
         assert result
         attr_string = result.group(1)
-        assert "width" in attr_string
-        assert "height" in attr_string
+        assert "width=" in attr_string
+        assert "height=" in attr_string
 
     def test_javascript_output(self):
         nb = v4.new_notebook(
@@ -103,13 +103,12 @@ def test_attachments(self):
         (output, resources) = HTMLExporter(template_name="classic").from_file(
             self._get_notebook(nb_name="attachment.ipynb")
         )
-        check_for_png = re.compile(r'<img src="[^"]*?"([^>]*?)>')
+        check_for_png = re.compile(r'<img alt="image.png" src="([^"]*?)"/>')
         result = check_for_png.search(output)
         assert result
-        self.assertTrue(result.group(0).strip().startswith('<img src="data:image/png;base64,iVBOR'))
-        self.assertTrue(result.group(1).strip().startswith('alt="image.png"'))
+        self.assertTrue(result.group(1).strip().startswith('data:image/png;base64,iVBOR'))
 
-        check_for_data = re.compile(r'<img src="(?P<url>[^"]*?)"')
+        check_for_data = re.compile(r'<img alt="image.png" src="(?P<url>[^"]*?)"')
         results = check_for_data.findall(output)
         assert results[0] != results[1], "attachments only need to be unique within a cell"
         assert "image/svg" in results[1], "second image should use svg"
diff --git a/nbconvert/tests/test_nbconvertapp.py b/nbconvert/tests/test_nbconvertapp.py
index ac005164f..2887211f9 100644
--- a/nbconvert/tests/test_nbconvertapp.py
+++ b/nbconvert/tests/test_nbconvertapp.py
@@ -335,13 +335,13 @@ def test_no_prompt(self):
             assert os.path.isfile("notebook1.html")
             with open("notebook1.html", encoding="utf8") as f:
                 text = f.read()
-                assert "In&nbsp;[" not in text
+                assert "In\xa0[" not in text
                 assert "Out[6]" not in text
             self.nbconvert("notebook1.ipynb --log-level 0 --to html")
             assert os.path.isfile("notebook1.html")
             with open("notebook1.html", encoding="utf8") as f:
                 text2 = f.read()
-                assert "In&nbsp;[" in text2
+                assert "In\xa0[" in text2
                 assert "Out[6]" in text2
 
     def test_cell_tag_output(self):
@@ -369,7 +369,7 @@ def test_no_input(self):
             '<span class="o">=</span> '
             '<span class="n">symbols</span>'
             '<span class="p">(</span>'
-            '<span class="s1">&#39;x y z&#39;</span>'
+            '<span class="s1">\'x y z\'</span>'
             '<span class="p">)</span>'
         )
         for no_input_flag in (False, True):
@@ -382,7 +382,7 @@ def test_no_input(self):
 
                 with open("notebook1.html", encoding="utf8") as f:
                     text = f.read()
-                    assert no_input_flag == ("In&nbsp;[" not in text)
+                    assert no_input_flag == ("In\xa0[" not in text)
                     assert no_input_flag == ("Out[6]" not in text)
                     assert no_input_flag == (input_content_html not in text)
 
@@ -580,7 +580,7 @@ def test_not_embedding_images_htmlexporter(self):
             with open("notebook5_embed_images.html", encoding="utf8") as f:
                 text = f.read()
                 assert "./containerized_deployments.jpeg" in text
-                assert "src='./containerized_deployments.jpeg'" in text
+                assert 'src="./containerized_deployments.jpeg"' in text
                 assert text.count("data:image/jpeg;base64") == 0
 
     def test_embedding_images_htmlexporter(self):
diff --git a/nbconvert/utils/iso639_1.py b/nbconvert/utils/iso639_1.py
new file mode 100644
index 000000000..ce5c2c664
--- /dev/null
+++ b/nbconvert/utils/iso639_1.py
@@ -0,0 +1,191 @@
+""" List of ISO639-1 language code"""
+
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+iso639_1 = [
+    "aa",
+    "ab",
+    "ae",
+    "af",
+    "ak",
+    "am",
+    "an",
+    "ar",
+    "as",
+    "av",
+    "ay",
+    "az",
+    "ba",
+    "be",
+    "bg",
+    "bh",
+    "bi",
+    "bm",
+    "bn",
+    "bo",
+    "br",
+    "bs",
+    "ca",
+    "ce",
+    "ch",
+    "co",
+    "cr",
+    "cs",
+    "cu",
+    "cv",
+    "cy",
+    "da",
+    "de",
+    "dv",
+    "dz",
+    "ee",
+    "el",
+    "en",
+    "eo",
+    "es",
+    "et",
+    "eu",
+    "fa",
+    "ff",
+    "fi",
+    "fj",
+    "fo",
+    "fr",
+    "fy",
+    "ga",
+    "gd",
+    "gl",
+    "gn",
+    "gu",
+    "gv",
+    "ha",
+    "he",
+    "hi",
+    "ho",
+    "hr",
+    "ht",
+    "hu",
+    "hy",
+    "hz",
+    "ia",
+    "id",
+    "ie",
+    "ig",
+    "ii",
+    "ik",
+    "io",
+    "is",
+    "it",
+    "iu",
+    "ja",
+    "jv",
+    "ka",
+    "kg",
+    "ki",
+    "kj",
+    "kk",
+    "kl",
+    "km",
+    "kn",
+    "ko",
+    "kr",
+    "ks",
+    "ku",
+    "kv",
+    "kw",
+    "ky",
+    "la",
+    "lb",
+    "lg",
+    "li",
+    "ln",
+    "lo",
+    "lt",
+    "lu",
+    "lv",
+    "mg",
+    "mh",
+    "mi",
+    "mk",
+    "ml",
+    "mn",
+    "mr",
+    "ms",
+    "mt",
+    "my",
+    "na",
+    "nb",
+    "nd",
+    "ne",
+    "ng",
+    "nl",
+    "nn",
+    "no",
+    "nr",
+    "nv",
+    "ny",
+    "oc",
+    "oj",
+    "om",
+    "or",
+    "os",
+    "pa",
+    "pi",
+    "pl",
+    "ps",
+    "pt",
+    "qu",
+    "rm",
+    "rn",
+    "ro",
+    "ru",
+    "rw",
+    "sa",
+    "sc",
+    "sd",
+    "se",
+    "sg",
+    "si",
+    "sk",
+    "sl",
+    "sm",
+    "sn",
+    "so",
+    "sq",
+    "sr",
+    "ss",
+    "st",
+    "su",
+    "sv",
+    "sw",
+    "ta",
+    "te",
+    "tg",
+    "th",
+    "ti",
+    "tk",
+    "tl",
+    "tn",
+    "to",
+    "tr",
+    "ts",
+    "tt",
+    "tw",
+    "ty",
+    "ug",
+    "uk",
+    "ur",
+    "uz",
+    "ve",
+    "vi",
+    "vo",
+    "wa",
+    "wo",
+    "xh",
+    "yi",
+    "yo",
+    "za",
+    "zh",
+    "zu",
+]
diff --git a/share/templates/classic/index.html.j2 b/share/templates/classic/index.html.j2
index 87a8578ac..30cd50264 100644
--- a/share/templates/classic/index.html.j2
+++ b/share/templates/classic/index.html.j2
@@ -4,7 +4,7 @@
 
 {%- block header -%}
 <!DOCTYPE html>
-<html>
+<html lang="{{ resources.language_code }}">
 <head>
 {%- block html_head -%}
 <meta charset="utf-8" />
@@ -92,13 +92,15 @@ div#notebook-container{
 
 {% block body_header %}
 <body>
-  <div tabindex="-1" id="notebook" class="border-box-sizing">
-    <div class="container" id="notebook-container">
+  <main>
+    <div tabindex="-1" id="notebook" class="border-box-sizing">
+      <div class="container" id="notebook-container">
 {% endblock body_header %}
 
 {% block body_footer %}
+      </div>
     </div>
-  </div>
+  </main>
 </body>
 {% endblock body_footer %}
 
diff --git a/share/templates/lab/index.html.j2 b/share/templates/lab/index.html.j2
index 7549a4538..ad184ca7c 100644
--- a/share/templates/lab/index.html.j2
+++ b/share/templates/lab/index.html.j2
@@ -5,7 +5,7 @@
 
 {%- block header -%}
 <!DOCTYPE html>
-<html>
+<html lang="{{ resources.language_code }}">
 <head>
 {%- block html_head -%}
 <meta charset="utf-8" />
@@ -125,9 +125,11 @@ a.anchor-link {
 {% else %}
 <body class="jp-Notebook" data-jp-theme-light="true" data-jp-theme-name="JupyterLab Light">
 {% endif %}
+<main>
 {%- endblock body_header -%}
 
 {% block body_footer %}
+</main>
 </body>
 {% endblock body_footer %}
 
diff --git a/share/templates/reveal/index.html.j2 b/share/templates/reveal/index.html.j2
index 84cb65280..26fed48c4 100644
--- a/share/templates/reveal/index.html.j2
+++ b/share/templates/reveal/index.html.j2
@@ -10,7 +10,7 @@
 
 {%- block header -%}
 <!DOCTYPE html>
-<html>
+<html lang="{{ resources.language_code }}">
 <head>
 
 {%- block html_head -%}
@@ -118,6 +118,7 @@ a.anchor-link {
 {% else %}
 <body class="jp-Notebook" data-jp-theme-light="true" data-jp-theme-name="JupyterLab Light">
 {% endif %}
+<main>
 <div class="reveal">
 <div class="slides">
 {%- endblock body_header -%}
@@ -125,6 +126,7 @@ a.anchor-link {
 {% block body_footer %}
 </div>
 </div>
+</main>
 </body>
 {% endblock body_footer %}