diff --git a/Libraries/LibWeb/Editing/Commands.cpp b/Libraries/LibWeb/Editing/Commands.cpp
index b82feb09e72..a059babdc31 100644
--- a/Libraries/LibWeb/Editing/Commands.cpp
+++ b/Libraries/LibWeb/Editing/Commands.cpp
@@ -20,6 +20,7 @@
 #include <LibWeb/HTML/HTMLImageElement.h>
 #include <LibWeb/HTML/HTMLLIElement.h>
 #include <LibWeb/HTML/HTMLTableElement.h>
+#include <LibWeb/HTML/Numbers.h>
 #include <LibWeb/Layout/Node.h>
 #include <LibWeb/Namespace.h>
 
@@ -500,6 +501,98 @@ bool command_font_name_action(DOM::Document& document, String const& value)
     return true;
 }
 
+enum class FontSizeMode : u8 {
+    Absolute,
+    RelativePlus,
+    RelativeMinus,
+};
+
+// https://w3c.github.io/editing/docs/execCommand/#the-fontsize-command
+bool command_font_size_action(DOM::Document& document, String const& value)
+{
+    // 1. Strip leading and trailing whitespace from value.
+    auto resulting_value = MUST(value.trim_ascii_whitespace());
+
+    // 2. If value is not a valid floating point number, and would not be a valid floating point number if a single
+    //    leading "+" character were stripped, return false.
+    if (!HTML::is_valid_floating_point_number(resulting_value)) {
+        if (!resulting_value.starts_with_bytes("+"sv)
+            || !HTML::is_valid_floating_point_number(MUST(resulting_value.substring_from_byte_offset(1))))
+            return false;
+    }
+
+    // 3. If the first character of value is "+", delete the character and let mode be "relative-plus".
+    auto mode = FontSizeMode::Absolute;
+    if (resulting_value.starts_with_bytes("+"sv)) {
+        resulting_value = MUST(resulting_value.substring_from_byte_offset(1));
+        mode = FontSizeMode::RelativePlus;
+    }
+
+    // 4. Otherwise, if the first character of value is "-", delete the character and let mode be "relative-minus".
+    else if (resulting_value.starts_with_bytes("-"sv)) {
+        resulting_value = MUST(resulting_value.substring_from_byte_offset(1));
+        mode = FontSizeMode::RelativeMinus;
+    }
+
+    // 5. Otherwise, let mode be "absolute".
+    // NOTE: This is the default set in step 3.
+
+    // 6. Apply the rules for parsing non-negative integers to value, and let number be the result.
+    i64 number = HTML::parse_non_negative_integer(resulting_value).release_value();
+
+    // 7. If mode is "relative-plus", add three to number.
+    if (mode == FontSizeMode::RelativePlus)
+        number += 3;
+
+    // 8. If mode is "relative-minus", negate number, then add three to it.
+    if (mode == FontSizeMode::RelativeMinus)
+        number = -number + 3;
+
+    // 9. If number is less than one, let number equal 1.
+    number = AK::max(number, 1);
+
+    // 10. If number is greater than seven, let number equal 7.
+    number = AK::min(number, 7);
+
+    // 11. Set value to the string here corresponding to number:
+    // 1: x-small
+    // 2: small
+    // 3: medium
+    // 4: large
+    // 5: x-large
+    // 6: xx-large
+    // 7: xxx-large
+    auto const& font_sizes = named_font_sizes();
+    resulting_value = MUST(String::from_utf8(font_sizes[number - 1]));
+
+    // 12. Set the selection's value to value.
+    set_the_selections_value(document, CommandNames::fontSize, resulting_value);
+
+    // 13. Return true.
+    return true;
+}
+
+// https://w3c.github.io/editing/docs/execCommand/#the-fontsize-command
+String command_font_size_value(DOM::Document const& document)
+{
+    // 1. If the active range is null, return the empty string.
+    auto range = active_range(document);
+    if (!range)
+        return {};
+
+    // 2. Let pixel size be the effective command value of the first formattable node that is effectively contained in
+    //    the active range, or if there is no such node, the effective command value of the active range's start node,
+    //    in either case interpreted as a number of pixels.
+    auto first_formattable_node = first_formattable_node_effectively_contained(range);
+    auto value = effective_command_value(
+        first_formattable_node ? first_formattable_node : *range->start_container(),
+        CommandNames::fontSize);
+    auto pixel_size = font_size_to_pixel_size(value.value_or({}));
+
+    // 3. Return the legacy font size for pixel size.
+    return legacy_font_size(pixel_size.to_int());
+}
+
 // https://w3c.github.io/editing/docs/execCommand/#the-forwarddelete-command
 bool command_forward_delete_action(DOM::Document& document, String const&)
 {
@@ -1129,6 +1222,13 @@ static Array const commands {
         .action = command_font_name_action,
         .relevant_css_property = CSS::PropertyID::FontFamily,
     },
+    // https://w3c.github.io/editing/docs/execCommand/#the-fontsize-command
+    CommandDefinition {
+        .command = CommandNames::fontSize,
+        .action = command_font_size_action,
+        .value = command_font_size_value,
+        .relevant_css_property = CSS::PropertyID::FontSize,
+    },
     // https://w3c.github.io/editing/docs/execCommand/#the-forwarddelete-command
     CommandDefinition {
         .command = CommandNames::forwardDelete,
diff --git a/Libraries/LibWeb/Editing/Commands.h b/Libraries/LibWeb/Editing/Commands.h
index 62df3b2d996..af971161fda 100644
--- a/Libraries/LibWeb/Editing/Commands.h
+++ b/Libraries/LibWeb/Editing/Commands.h
@@ -36,6 +36,8 @@ bool command_default_paragraph_separator_action(DOM::Document&, String const&);
 String command_default_paragraph_separator_value(DOM::Document const&);
 bool command_delete_action(DOM::Document&, String const&);
 bool command_font_name_action(DOM::Document&, String const&);
+bool command_font_size_action(DOM::Document&, String const&);
+String command_font_size_value(DOM::Document const&);
 bool command_forward_delete_action(DOM::Document&, String const&);
 bool command_insert_linebreak_action(DOM::Document&, String const&);
 bool command_insert_paragraph_action(DOM::Document&, String const&);
diff --git a/Libraries/LibWeb/Editing/Internal/Algorithms.cpp b/Libraries/LibWeb/Editing/Internal/Algorithms.cpp
index 1597657f6ce..5d0bc7900a4 100644
--- a/Libraries/LibWeb/Editing/Internal/Algorithms.cpp
+++ b/Libraries/LibWeb/Editing/Internal/Algorithms.cpp
@@ -3961,7 +3961,12 @@ GC::Ptr<DOM::Node> first_formattable_node_effectively_contained(GC::Ptr<DOM::Ran
 
 CSSPixels font_size_to_pixel_size(StringView font_size)
 {
-    auto pixel_size = CSS::StyleComputer::default_user_font_size();
+    // If the font size ends in 'px', interpret the preceding as a number and return it.
+    if (font_size.length() >= 2 && font_size.substring_view(font_size.length() - 2).equals_ignoring_ascii_case("px"sv)) {
+        auto optional_number = font_size.substring_view(0, font_size.length() - 2).to_number<float>();
+        if (optional_number.has_value())
+            return CSSPixels::nearest_value_for(optional_number.value());
+    }
 
     // Try to map the font size directly to a keyword (e.g. medium or x-large)
     auto keyword = CSS::keyword_from_string(font_size);
@@ -3971,6 +3976,7 @@ CSSPixels font_size_to_pixel_size(StringView font_size)
         keyword = HTML::HTMLFontElement::parse_legacy_font_size(font_size);
 
     // If that also failed, give up
+    auto pixel_size = CSS::StyleComputer::default_user_font_size();
     if (!keyword.has_value())
         return pixel_size;
 
diff --git a/Tests/LibWeb/Text/expected/Editing/execCommand-fontSize.txt b/Tests/LibWeb/Text/expected/Editing/execCommand-fontSize.txt
new file mode 100644
index 00000000000..c70bbba9c28
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/Editing/execCommand-fontSize.txt
@@ -0,0 +1,6 @@
+div: "foobar"
+fontSize value: 3
+div: "foo<font size="5" style="font-size: x-large;">bar</font>"
+fontSize value: 5
+div: "<font size="4" style="font-size: large;">foo</font><font style=""><font size="4" style="font-size: large;"><font size="4" style="font-size: large;">bar</font></font></font>"
+fontSize value: 4
diff --git a/Tests/LibWeb/Text/input/Editing/execCommand-fontSize.html b/Tests/LibWeb/Text/input/Editing/execCommand-fontSize.html
new file mode 100644
index 00000000000..f8508dbf870
--- /dev/null
+++ b/Tests/LibWeb/Text/input/Editing/execCommand-fontSize.html
@@ -0,0 +1,26 @@
+<script src="../include.js"></script>
+<div contenteditable="true">foobar</div>
+<script>
+    test(() => {
+        const range = document.createRange();
+        getSelection().addRange(range);
+
+        const divElm = document.querySelector('div');
+        println(`div: "${divElm.innerHTML}"`);
+        println(`fontSize value: ${document.queryCommandValue('fontSize')}`);
+
+        // Set fontSize for 'bar'
+        range.setStart(divElm.childNodes[0], 3);
+        range.setEnd(divElm.childNodes[0], 6);
+        document.execCommand('fontSize', false, '5');
+        println(`div: "${divElm.innerHTML}"`);
+        println(`fontSize value: ${document.queryCommandValue('fontSize')}`);
+
+        // Set fontSize for 'foobar'
+        range.setStart(divElm.childNodes[0], 0);
+        range.setEnd(divElm.childNodes[1].childNodes[0], 3);
+        document.execCommand('fontSize', false, '4');
+        println(`div: "${divElm.innerHTML}"`);
+        println(`fontSize value: ${document.queryCommandValue('fontSize')}`);
+    });
+</script>