diff --git a/.changeset/fix-html-lower-case.md b/.changeset/fix-html-lower-case.md
new file mode 100644
index 00000000..56fc6e61
--- /dev/null
+++ b/.changeset/fix-html-lower-case.md
@@ -0,0 +1,5 @@
+---
+"preact-render-to-string": patch
+---
+
+Replace `HTML_LOWER_CASE` regex with an explicit `Set` to prevent false matches on custom element properties (e.g. `channelId` being incorrectly lowercased to `channelid`)
diff --git a/src/index.js b/src/index.js
index f1bf5b93..39b267c2 100644
--- a/src/index.js
+++ b/src/index.js
@@ -300,6 +300,8 @@ function _renderToString(
 
 	let type = vnode.type,
 		props = vnode.props;
+	
+	const isCustomElement = type.includes("-");
 
 	// Invoke rendering on Components
 	if (typeof type == 'function') {
@@ -666,7 +668,7 @@ function _renderToString(
 								? 'panose-1'
 								: name.replace(/([A-Z])/g, '-$1').toLowerCase();
 					}
-				} else if (HTML_LOWER_CASE.test(name)) {
+				} else if (!isCustomElement && HTML_LOWER_CASE.has(name)) {
 					name = name.toLowerCase();
 				}
 			}
diff --git a/src/lib/util.js b/src/lib/util.js
index 4be4e192..f4c9e817 100644
--- a/src/lib/util.js
+++ b/src/lib/util.js
@@ -1,10 +1,54 @@
 import { DIRTY, BITS } from './constants';
 
 export const VOID_ELEMENTS = /^(?:area|base|br|col|embed|hr|img|input|link|meta|param|source|track|wbr)$/;
+
 // oxlint-disable-next-line no-control-regex
 export const UNSAFE_NAME = /[\s\n\\/='"\0<>]/;
 export const NAMESPACE_REPLACE_REGEX = /^(xlink|xmlns|xml)([A-Z])/;
-export const HTML_LOWER_CASE = /^(?:accessK|auto[A-Z]|cell|ch|col|cont|cross|dateT|encT|form[A-Z]|frame|hrefL|inputM|maxL|minL|noV|playsI|popoverT|readO|rowS|src[A-Z]|tabI|useM|item[A-Z])/;
+export const HTML_LOWER_CASE = new Set([
+	'accessKey',
+	'autoCapitalize',
+	'autoComplete',
+	'autoCorrect',
+	'autoFocus',
+	'autoPlay',
+	'cellPadding',
+	'cellSpacing',
+	'charSet',
+	'colSpan',
+	'contentEditable',
+	'contextMenu',
+	'controlsList',
+	'crossOrigin',
+	'dateTime',
+	'encType',
+	'formAction',
+	'formEncType',
+	'formMethod',
+	'formNoValidate',
+	'formTarget',
+	'frameBorder',
+	'hrefLang',
+	'inputMode',
+	'maxLength',
+	'minLength',
+	'noValidate',
+	'playsInline',
+	'popoverTarget',
+	'popoverTargetAction',
+	'readOnly',
+	'rowSpan',
+	'srcDoc',
+	'srcLang',
+	'srcSet',
+	'tabIndex',
+	'useMap',
+	'itemScope',
+	'itemType',
+	'itemID',
+	'itemRef',
+	'itemProp'
+]);
 export const SVG_CAMEL_CASE = /^ac|^ali|arabic|basel|cap|clipPath$|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/;
 
 // Boolean DOM properties that translate to enumerated ('true'/'false') attributes
diff --git a/src/pretty.js b/src/pretty.js
index fe630744..f9c62e9c 100644
--- a/src/pretty.js
+++ b/src/pretty.js
@@ -129,6 +129,8 @@ function _renderToStringPretty(
 		props = vnode.props,
 		isComponent = false;
 
+	const isCustomElement = nodeName.includes("-");
+
 	// components
 	if (typeof nodeName === 'function') {
 		isComponent = true;
@@ -299,7 +301,7 @@ function _renderToStringPretty(
 							? 'panose-1'
 							: name.replace(/([A-Z])/g, '-$1').toLowerCase();
 				}
-			} else if (HTML_LOWER_CASE.test(name)) {
+			} else if (!isCustomElement && HTML_LOWER_CASE.has(name)) {
 				name = name.toLowerCase();
 			}