diff --git a/ext/dom/lexbor/lexbor/selectors-adapted/selectors.c b/ext/dom/lexbor/lexbor/selectors-adapted/selectors.c
index 8bd996a3910fe..3a40318628f10 100644
--- a/ext/dom/lexbor/lexbor/selectors-adapted/selectors.c
+++ b/ext/dom/lexbor/lexbor/selectors-adapted/selectors.c
@@ -65,7 +65,21 @@ static zend_always_inline bool lxb_selectors_adapted_cmp_local_name_id(const xml
 
 static zend_always_inline const xmlAttr *lxb_selectors_adapted_attr(const xmlNode *node, const lxb_char_t *name)
 {
-	const xmlAttr *attr = xmlHasProp(node, (const xmlChar *) name);
+	const xmlAttr *attr = NULL;
+	ZEND_ASSERT(node->doc != NULL);
+	if (php_dom_ns_is_html_and_document_is_html(node)) {
+		/* No need to handle DTD entities as we're in HTML. */
+		size_t name_bound = strlen((const char *) name) + 1;
+		for (const xmlAttr *cur = node->properties; cur != NULL; cur = cur->next) {
+			if (lexbor_str_data_nlocmp_right(cur->name, name, name_bound)) {
+				attr = cur;
+				break;
+			}
+		}
+	} else {
+		attr = xmlHasProp(node, (const xmlChar *) name);
+	}
+
 	if (attr != NULL && attr->ns != NULL) {
 		return NULL;
 	}
@@ -85,8 +99,67 @@ static zend_always_inline dom_lxb_str_wrapper lxb_selectors_adapted_attr_value(c
 	return ret;
 }
 
+static bool lxb_selectors_attrib_name_cmp(const lxb_css_selector_t *selector, const char *name, size_t len)
+{
+	return selector->name.length == len && lexbor_str_data_nlocmp_right((const lxb_char_t *) name, selector->name.data, len);
+}
+
+/* From https://html.spec.whatwg.org/#case-sensitivity-of-selectors
+ * "Attribute selectors on an HTML element in an HTML document must treat the values of attributes with the following names as ASCII case-insensitive:" */
+static bool lxb_selectors_is_lowercased_html_attrib_name(const lxb_css_selector_t *selector)
+{
+	return lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("accept"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("accept-charset"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("align"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("alink"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("axis"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("bgcolor"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("charset"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("checked"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("clear"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("codetype"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("color"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("compact"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("declare"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("defer"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("dir"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("direction"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("disabled"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("enctype"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("face"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("frame"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("hreflang"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("http-equiv"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("lang"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("language"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("link"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("media"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("method"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("multiple"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("nohref"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("noresize"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("noshade"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("nowrap"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("readonly"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("rel"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("rev"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("rules"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("scope"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("scrolling"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("selected"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("shape"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("target"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("text"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("type"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("valign"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("valuetype"))
+		|| lxb_selectors_attrib_name_cmp(selector, ZEND_STRL("vlink"));
+}
+
 static void lxb_selectors_adapted_set_entry_id_ex(lxb_selectors_entry_t *entry, const lxb_css_selector_t *selector, const xmlNode *node)
 {
+	entry->id.attr_case_insensitive = lxb_selectors_is_lowercased_html_attrib_name(selector);
+
 	if (node->doc != NULL && node->doc->dict != NULL) {
 		const xmlChar *interned = xmlDictExists(node->doc->dict, selector->name.data, selector->name.length);
 		if (interned != NULL) {
@@ -1290,10 +1363,10 @@ lxb_selectors_match_class(const lexbor_str_t *target, const lexbor_str_t *src,
 }
 
 static bool
-lxb_selectors_match_attribute_value(const lxb_css_selector_attribute_t *attr, const lexbor_str_t *trg, const lexbor_str_t *src)
+lxb_selectors_match_attribute_value(const lxb_css_selector_attribute_t *attr, bool force_modifier_i, const lexbor_str_t *trg, const lexbor_str_t *src)
 {
 	bool res;
-	bool ins = attr->modifier == LXB_CSS_SELECTOR_MODIFIER_I;
+	bool ins = attr->modifier == LXB_CSS_SELECTOR_MODIFIER_I || force_modifier_i;
 
 	switch (attr->match) {
 		case LXB_CSS_SELECTOR_MATCH_EQUAL:      /*  = */
@@ -1405,7 +1478,13 @@ lxb_selectors_match_attribute(const lxb_css_selector_t *selector,
 	}
 
 	dom_lxb_str_wrapper trg = lxb_selectors_adapted_attr_value(dom_attr);
-	bool res = lxb_selectors_match_attribute_value(attr, &trg.str, src);
+	ZEND_ASSERT(node->doc != NULL);
+	bool res = lxb_selectors_match_attribute_value(
+		attr,
+		entry->id.attr_case_insensitive && php_dom_ns_is_html_and_document_is_html(node),
+		&trg.str,
+		src
+	);
 	dom_lxb_str_wrapper_release(&trg);
 	return res;
 }
diff --git a/ext/dom/lexbor/lexbor/selectors-adapted/selectors.h b/ext/dom/lexbor/lexbor/selectors-adapted/selectors.h
index 441976b1e3d41..9057fae684189 100644
--- a/ext/dom/lexbor/lexbor/selectors-adapted/selectors.h
+++ b/ext/dom/lexbor/lexbor/selectors-adapted/selectors.h
@@ -78,6 +78,7 @@ typedef lxb_selectors_entry_t *
 typedef struct {
 	const xmlChar *name;
 	bool interned;
+	bool attr_case_insensitive;
 } lxb_selectors_adapted_id;
 
 struct lxb_selectors_entry {
diff --git a/ext/dom/tests/modern/css_selectors/gh17802.phpt b/ext/dom/tests/modern/css_selectors/gh17802.phpt
new file mode 100644
index 0000000000000..2af1dab622836
--- /dev/null
+++ b/ext/dom/tests/modern/css_selectors/gh17802.phpt
@@ -0,0 +1,63 @@
+--TEST--
+GH-17802 (\Dom\HTMLDocument querySelector attribute name is case sensitive in HTML)
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+
+$text = <<<TEXT
+<html>
+<head>
+<meta charset="Windows-1252">
+</head>
+<body>
+</body>
+</html>
+TEXT;
+
+$dom = \Dom\HTMLDocument::createFromString($text, options: LIBXML_NOERROR);
+$meta2 = $dom->head->appendChild($dom->createElementNS('urn:x', 'meta'));
+$meta2->setAttribute('charset', 'x');
+echo $dom->saveHtml(), "\n";
+
+echo "--- charseT ---\n";
+
+foreach ($dom->querySelectorAll('meta[charseT]') as $entry) {
+    var_dump($dom->saveHtml($entry));
+}
+
+echo "--- charset ---\n";
+
+foreach ($dom->querySelectorAll('meta[charset]') as $entry) {
+    var_dump($dom->saveHtml($entry));
+}
+
+echo "--- charseT and lowercase value ---\n";
+
+foreach ($dom->querySelectorAll('meta[charseT="windows-1252"]') as $entry) {
+    var_dump($dom->saveHtml($entry));
+}
+
+echo "--- charset and lowercase value ---\n";
+
+foreach ($dom->querySelectorAll('meta[charset="windows-1252"]') as $entry) {
+    var_dump($dom->saveHtml($entry));
+}
+
+?>
+--EXPECT--
+<html><head>
+<meta charset="Windows-1252">
+<meta charset="x"></meta></head>
+<body>
+
+</body></html>
+--- charseT ---
+string(29) "<meta charset="Windows-1252">"
+--- charset ---
+string(29) "<meta charset="Windows-1252">"
+string(25) "<meta charset="x"></meta>"
+--- charseT and lowercase value ---
+string(29) "<meta charset="Windows-1252">"
+--- charset and lowercase value ---
+string(29) "<meta charset="Windows-1252">"