diff --git a/.changeset/blue-moons-behave.md b/.changeset/blue-moons-behave.md
new file mode 100644
index 00000000..497be8a8
--- /dev/null
+++ b/.changeset/blue-moons-behave.md
@@ -0,0 +1,5 @@
+---
+'preact-render-to-string': patch
+---
+
+Ensure `popoverTarget` and `popoverTargetAction` are serialized to lower case
diff --git a/src/lib/util.js b/src/lib/util.js
index 55fc0b1e..12b5a4cc 100644
--- a/src/lib/util.js
+++ b/src/lib/util.js
@@ -1,7 +1,7 @@
 export const VOID_ELEMENTS = /^(?:area|base|br|col|embed|hr|img|input|link|meta|param|source|track|wbr)$/;
 export const UNSAFE_NAME = /[\s\n\\/='"\0<>]/;
 export const NAMESPACE_REPLACE_REGEX = /^(xlink|xmlns|xml)([A-Z])/;
-export const HTML_LOWER_CASE = /^accessK|^auto[A-Z]|^ch|^col|cont|cross|dateT|encT|form[A-Z]|frame|hrefL|inputM|maxL|minL|noV|playsI|readO|rowS|spellC|src[A-Z]|tabI|item[A-Z]/;
+export const HTML_LOWER_CASE = /^accessK|^auto[A-Z]|^ch|^col|cont|cross|dateT|encT|form[A-Z]|frame|hrefL|inputM|maxL|minL|noV|playsI|popoverT|readO|rowS|spellC|src[A-Z]|tabI|item[A-Z]/;
 export const SVG_CAMEL_CASE = /^ac|^ali|arabic|basel|cap|clipPath$|clipRule$|color|dominant|enable|fill|flood|font|glyph[^R]|horiz|image|letter|lighting|marker[^WUH]|overline|panose|pointe|paint|rendering|shape|stop|strikethrough|stroke|text[^L]|transform|underline|unicode|units|^v[^i]|^w|^xH/;
 
 // DOM properties that should NOT have "px" added when numeric
diff --git a/test/utils.js b/test/utils.js
index d0438a34..cadfb55a 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -375,6 +375,8 @@ export const htmlAttributes = {
 	ping: 'ping',
 	placeholder: 'placeholder',
 	playsInline: 'playsinline',
+	popoverTarget: 'popovertarget',
+	popoverTargetAction: 'popovertargetaction',
 	poster: 'poster',
 	preload: 'preload',
 	readonly: 'readonly',