diff --git a/doc/lua_api.md b/doc/lua_api.md
index 3cf586faa..b46973fba 100644
--- a/doc/lua_api.md
+++ b/doc/lua_api.md
@@ -2681,6 +2681,8 @@ Version History
 * Formspec version 7 (5.8.0):
   * style[]: Add focused state for buttons
   * Add field_enter_after_edit[] (experimental)
+* Formspec version 8 (5.9.0):
+  * hypertext[]: Allow `em` as a length unit
 
 Elements
 --------
@@ -3535,6 +3537,10 @@ Some tags can enclose text, they open with `<tagname>` and close with `</tagname
 Tags can have attributes, in that case, attributes are in the opening tag in
 form of a key/value separated with equal signs. Attribute values should not be quoted.
 
+Attributes that describe lengths (font size and image dimensions) can be a regular
+number, which describes the size in pixels, or a number with the unit `em`, which
+describes the size in relation to the font size of the enclosing element.
+
 If you want to insert a literal greater-than sign or a backslash into the text,
 you must escape it by preceding it with a backslash.
 
diff --git a/games/devtest/mods/testformspec/formspec.lua b/games/devtest/mods/testformspec/formspec.lua
index 758bad631..5734f9a6e 100644
--- a/games/devtest/mods/testformspec/formspec.lua
+++ b/games/devtest/mods/testformspec/formspec.lua
@@ -70,7 +70,9 @@ This is a normal text.
 
 <bigger><mono>style</mono> test</bigger>
 <style color=#FFFF00>Yellow text.</style> <style color=#FF0000>Red text.</style>
-<style size=24>Size 24.</style> <style size=16>Size 16</style>. <style size=12>Size 12.</style>
+<style size=24>Size 24.</style> <style size=16>Size 16.</style> <style size=12>Size 12.</style>
+<style size=2em>Size 2em.</style> <style size=1em>Size 1em.</style> <style size=0.5em>Size 0.5em.</style>
+<style size=3em>Size 3em. <style size=0.5em>Half of 3em.</style></style>
 <style font=normal>Normal font.</style> <style font=mono>Mono font.</style>
 
 <bigger>Tag test</bigger>
@@ -112,6 +114,10 @@ Normal:
 <img name=testformspec_item.png float=left>
 <mono>float=right</mono>:
 <img name=testformspec_item.png float=right>
+<mono>width=1em height=2em</mono>:
+<img name=testformspec_item.png width=1em height=2em>
+<mono>width=2em</mono> inside <mono>width=2em</mono> (should render at <mono>width=4em</mono>):
+<style size=2em><img name=testformspec_item.png width=2em></style>
 
 <bigger><mono>item</mono> test</bigger>
 Normal:
diff --git a/src/gui/guiHyperText.cpp b/src/gui/guiHyperText.cpp
index 76bc98a71..12d78dc5b 100644
--- a/src/gui/guiHyperText.cpp
+++ b/src/gui/guiHyperText.cpp
@@ -49,6 +49,49 @@ static bool check_integer(const std::string &str)
 	return *endptr == '\0';
 }
 
+static bool check_length(const std::string &str)
+{
+	return ParsedText::LengthValue(str).isValid();
+}
+
+// -----------------------------------------------------------------------------
+// LengthValue - A data structure representing a (possibly relative) length
+
+ParsedText::LengthValue::LengthValue(const std::string &str)
+{
+	char *unitptr = nullptr;
+	this->size = std::strtod(str.c_str(), &unitptr);
+
+	if (this->size <= 0)
+		this->size = 0;
+	this->unit = unitptr;
+}
+
+bool ParsedText::LengthValue::isValid()
+{
+	return this->getAbsoluteValue(1) > 0;
+}
+
+double ParsedText::LengthValue::getAbsoluteValue(const double &em_size)
+{
+	if (this->unit.empty())
+		return this->size;
+	else if (this->unit == "em")
+		return this->size * em_size;
+	else
+		return 0;
+}
+
+double ParsedText::LengthValue::getAbsoluteValue()
+{
+	return getAbsoluteValue(0);
+}
+
+bool ParsedText::LengthValue::isAbsolute()
+{
+	return getAbsoluteValue() > 0;
+}
+
 // -----------------------------------------------------------------------------
 // ParsedText - A text parser
 
@@ -63,7 +106,7 @@ void ParsedText::Element::setStyle(StyleList &style)
 	if (parseColorString(style["hovercolor"], color, false))
 		this->hovercolor = color;
 
-	unsigned int font_size = std::atoi(style["fontsize"].c_str());
+	this->font_size = std::atoi(style["fontsize"].c_str());
 
 	FontMode font_mode = FM_Standard;
 	if (style["fontstyle"] == "mono")
@@ -72,9 +115,9 @@ void ParsedText::Element::setStyle(StyleList &style)
 	// hypertext[] only accepts absolute font size values and has a hardcoded
 	// default font size of 16. This is the only way to make hypertext[]
 	// respect font size settings that I can think of.
-	font_size = myround(font_size / 16.0f * g_fontengine->getFontSize(font_mode));
+	this->font_size = myround(this->font_size / 16.0f * g_fontengine->getFontSize(font_mode));
 
-	FontSpec spec(font_size, font_mode,
+	FontSpec spec(this->font_size, font_mode,
 		is_yes(style["bold"]), is_yes(style["italic"]));
 
 	// TODO: find a way to check font validity
@@ -83,7 +126,7 @@ void ParsedText::Element::setStyle(StyleList &style)
 
 	if (!this->font)
 		printf("No font found ! Size=%d, mode=%d, bold=%s, italic=%s\n",
-				font_size, font_mode, style["bold"].c_str(),
+				this->font_size, font_mode, style["bold"].c_str(),
 				style["italic"].c_str());
 }
 
@@ -341,9 +384,8 @@ void ParsedText::parseGenericStyleAttr(
 		style[name] = is_yes(value);
 
 	} else if (name == "size") {
-		if (check_integer(value))
+		if (check_length(value))
 			style["fontsize"] = value;
-
 	} else if (name == "font") {
 		if (value == "mono" || value == "normal")
 			style["fontstyle"] = value;
@@ -512,13 +554,13 @@ u32 ParsedText::parseTag(const wchar_t *text, u32 cursor)
 		}
 
 		if (attrs.count("width")) {
-			int width = stoi(attrs["width"]);
+			int width = LengthValue(attrs["width"]).getAbsoluteValue(m_element->font_size);
 			if (width > 0)
 				m_element->dim.Width = width;
 		}
 
 		if (attrs.count("height")) {
-			int height = stoi(attrs["height"]);
+			int height = LengthValue(attrs["height"]).getAbsoluteValue(m_element->font_size);
 			if (height > 0)
 				m_element->dim.Height = height;
 		}
@@ -595,9 +637,18 @@ u32 ParsedText::parseTag(const wchar_t *text, u32 cursor)
 
 	// Update styles accordingly
 	m_style.clear();
-	for (auto tag = m_active_tags.crbegin(); tag != m_active_tags.crend(); ++tag)
-		for (const auto &prop : (*tag)->style)
-			m_style[prop.first] = prop.second;
+	unsigned int font_size = 0;
+	for (auto tag = m_active_tags.crbegin(); tag != m_active_tags.crend(); ++tag) {
+		for (const auto &prop : (*tag)->style) {
+			if (prop.first == "fontsize") {
+				// resolve font size
+				font_size = LengthValue(prop.second).getAbsoluteValue(font_size);
+			} else {
+				m_style[prop.first] = prop.second;
+			}
+		}
+	}
+	m_style["fontsize"] = std::to_string(font_size);
 
 	return cursor;
 }
diff --git a/src/gui/guiHyperText.h b/src/gui/guiHyperText.h
index 0616a37ce..bc0344b54 100644
--- a/src/gui/guiHyperText.h
+++ b/src/gui/guiHyperText.h
@@ -76,6 +76,17 @@ public:
 	typedef std::unordered_map<std::string, std::string> StyleList;
 	typedef std::unordered_map<std::string, std::string> AttrsList;
 
+	struct LengthValue
+	{
+		double size;
+		std::string unit;
+		LengthValue(const std::string &str);
+		bool isValid();
+		bool isAbsolute();
+		double getAbsoluteValue();
+		double getAbsoluteValue(const double &em_size);
+	};
+
 	struct Tag
 	{
 		std::string name;
@@ -98,6 +109,7 @@ public:
 		ValignType valign;
 
 		gui::IGUIFont *font;
+		unsigned int font_size;
 
 		irr::video::SColor color;
 		irr::video::SColor hovercolor;
diff --git a/src/network/networkprotocol.h b/src/network/networkprotocol.h
index add80b3b2..e1ea47b21 100644
--- a/src/network/networkprotocol.h
+++ b/src/network/networkprotocol.h
@@ -244,7 +244,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
                             // base64-encoded SHA-1 (27+\0).
 
 // See also formspec [Version History] in doc/lua_api.md
-#define FORMSPEC_API_VERSION 7
+#define FORMSPEC_API_VERSION 8
 
 #define TEXTURENAME_ALLOWED_CHARS "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_.-"