diff --git a/NEWS b/NEWS
index 7fa72d486b5..c6bfbc62479 100644
--- a/NEWS
+++ b/NEWS
@@ -21,6 +21,8 @@ PHP                                                                        NEWS
 - DOM:
   . Fixed bug GH-17609 (Typo in error message: Dom\NO_DEFAULT_NS instead of
     Dom\HTML_NO_DEFAULT_NS). (nielsdos)
+  . Fixed bug GH-17802 (\Dom\HTMLDocument querySelector attribute name is case
+    sensitive in HTML). (nielsdos)
 
 - GD:
   . Fixed bug GH-17703 (imagescale with both width and height negative values
diff --git a/ext/dom/lexbor/lexbor/selectors-adapted/selectors.c b/ext/dom/lexbor/lexbor/selectors-adapted/selectors.c
index 7d99ee8e0bc..3a40318628f 100644
--- a/ext/dom/lexbor/lexbor/selectors-adapted/selectors.c
+++ b/ext/dom/lexbor/lexbor/selectors-adapted/selectors.c
@@ -99,8 +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) {
@@ -1304,10 +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:      /*  = */
@@ -1419,7 +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 441976b1e3d..9057fae6841 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
index 4797b7fb1c9..2af1dab6228 100644
--- a/ext/dom/tests/modern/css_selectors/gh17802.phpt
+++ b/ext/dom/tests/modern/css_selectors/gh17802.phpt
@@ -32,6 +32,18 @@ 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>
@@ -45,3 +57,7 @@ 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">"