diff --git a/mdit_py_plugins/attrs/index.py b/mdit_py_plugins/attrs/index.py
index 0f608c8..124b6d0 100644
--- a/mdit_py_plugins/attrs/index.py
+++ b/mdit_py_plugins/attrs/index.py
@@ -41,8 +41,14 @@ def attrs_plugin(
     - `key="value"` or `key=value` specifies a key-value attribute.
        Quotes are not needed when the value consists entirely of
        ASCII alphanumeric characters or `_` or `:` or `-`.
-       Backslash escapes may be used inside quoted values.
-    - `%` begins a comment, which ends with the next `%` or the end of the attribute (`}`).
+       Backslash escapes may be used inside quoted values,
+       to allow a ``"`` character within the value.
+       Note that the backslash is retained in the value:
+       ``{k="a\\"b"}`` gives ``k`` the value ``a\\"b``.
+    - `%` begins a comment, which ends with the next `%`.
+      If no further `%` occurs in the rest of the line (for a block attribute)
+      or of the paragraph (for an inline attribute),
+      the comment instead ends at the next `}`, which also ends the attribute.
 
     Multiple attribute blocks are merged.
 
@@ -175,8 +181,8 @@ def _attr_inline_rule(
     state.pos += new_pos + 1
     if not silent:
         attr_token = state.tokens[token_index]
-        if "class" in attrs and "class" in token.attrs:
-            attrs["class"] = f"{token.attrs['class']} {attrs['class']}"
+        if "class" in attrs and "class" in attr_token.attrs:
+            attrs["class"] = f"{attr_token.attrs['class']} {attrs['class']}"
         _add_attrs(attr_token, attrs, allowed)
     return True
 
@@ -238,7 +244,8 @@ def _attr_resolve_block_rule(state: StateCore, *, allowed: set[str] | None) -> N
             i += 1
             continue
 
-        if i + 1 < len_tokens:
+        # skip closing tokens; attributes would be rendered into the closing tag
+        if i + 1 < len_tokens and state.tokens[i + 1].nesting >= 0:
             next_token = state.tokens[i + 1]
 
             # classes are appended
diff --git a/mdit_py_plugins/attrs/parse.py b/mdit_py_plugins/attrs/parse.py
index 061574f..605bbfb 100644
--- a/mdit_py_plugins/attrs/parse.py
+++ b/mdit_py_plugins/attrs/parse.py
@@ -100,17 +100,28 @@ def parse(string: str) -> tuple[int, dict[str, str]]:
     """Parse attributes from start of string.
 
     :returns: (length of parsed string, dict of attributes)
+    :raises ParseError: if the attributes are malformed,
+        or the string ends before the closing `}`
     """
     pos = 0
     state: State = State.START
     tokens = TokenState()
     while pos < len(string):
-        state = HANDLERS[state](string[pos], pos, tokens)
+        if (
+            state == State.SCANNING_COMMENT
+            and string[pos] == "}"
+            and "%" not in string[pos + 1 :]
+        ):
+            # a comment ends at the next `%`, but there is none left to close it,
+            # so this `}` ends both the comment and the attributes
+            state = State.DONE
+        else:
+            state = HANDLERS[state](string[pos], pos, tokens)
         if state == State.DONE:
             return pos, tokens.compile(string)
         pos = pos + 1
 
-    return pos, tokens.compile(string)
+    raise ParseError("Attributes not terminated", pos)
 
 
 def handle_start(char: str, pos: int, tokens: TokenState) -> State:
diff --git a/tests/fixtures/attrs.md b/tests/fixtures/attrs.md
index b6d3890..6523b94 100644
--- a/tests/fixtures/attrs.md
+++ b/tests/fixtures/attrs.md
@@ -61,6 +61,61 @@ a = 1
 </code></pre>
 .
 
+block: attrs last in a blockquote
+.
+> {.a}
+
+para
+.
+<blockquote></blockquote>
+<p>para</p>
+.
+
+block: attrs last in a list item
+.
+- {.a}
+.
+<ul>
+<li></li>
+</ul>
+.
+
+block: attrs last in an ordered list item
+.
+1. {.a}
+.
+<ol>
+<li></li>
+</ol>
+.
+
+block: two attrs blocks last in a blockquote
+.
+> {.a}
+> {.b}
+.
+<blockquote></blockquote>
+.
+
+block: attrs before a blockquote containing only attrs
+.
+{.a}
+{.b}
+> {.c}
+.
+<blockquote class="a b"></blockquote>
+.
+
+block: attrs followed by a paragraph in a blockquote
+.
+> {.a}
+> para
+.
+<blockquote>
+<p class="a">para</p>
+</blockquote>
+.
+
 block after paragraph
 .
 a
@@ -70,6 +125,64 @@ a
 {#a .a c=1}</p>
 .
 
+unterminated: block group is not an attribute block
+.
+{.a
+para
+.
+<p>{.a
+para</p>
+.
+
+comment: block
+.
+{%x}
+para
+.
+<p>para</p>
+.
+
+comment: block after a class
+.
+{.a %c}
+para
+.
+<p class="a">para</p>
+.
+
+comment: block closed by a percent
+.
+{% just a comment %}
+para
+.
+<p>para</p>
+.
+
+comment: block with a brace inside the comment
+.
+{% x } y %}
+para
+.
+<p>para</p>
+.
+
+comment: block, attributes after a brace inside the comment
+.
+{% c } % .b}
+para
+.
+<p class="b">para</p>
+.
+
+comment: block, a brace inside the comment does not terminate the group
+.
+{% see } below %{x}
+para
+.
+<p>{% see } below %{x}
+para</p>
+.
+
 
 simple reference link
 .
@@ -101,6 +214,13 @@ simple inline code
 <p><code id="id" b="c" class="a">a</code></p>
 .
 
+quoted value keeps a backslash escape
+.
+`a`{k="a\"b"}
+.
+<p><code k="a\&quot;b">a</code></p>
+.
+
 ignore if space
 .
 ![a](b) {#id key="*"}
@@ -134,6 +254,134 @@ merging attributes
 <p><img src="b" alt="a" id="x" class="a b x x g" other="a"></p>
 .
 
+unterminated: inline code
+.
+`a`{
+.
+<p><code>a</code>{</p>
+.
+
+unterminated: inline code with a class
+.
+`a`{.a
+.
+<p><code>a</code>{.a</p>
+.
+
+unterminated: link
+.
+[a](u){
+.
+<p><a href="u">a</a>{</p>
+.
+
+unterminated: image
+.
+![a](u){.x
+.
+<p><img src="u" alt="a">{.x</p>
+.
+
+unterminated: following text is not consumed
+.
+`a`{.a
+more
+.
+<p><code>a</code>{.a
+more</p>
+.
+
+unterminated: group after a terminated group
+.
+`a`{.a}{
+.
+<p><code class="a">a</code>{</p>
+.
+
+unterminated: partially scanned attributes are not applied
+.
+`a`{ .a b
+.
+<p><code>a</code>{ .a b</p>
+.
+
+unterminated: escaped brace inside a quoted value
+.
+`a`{k="a\}
+.
+<p><code>a</code>{k=&quot;a}</p>
+.
+
+unterminated: inside a link label
+.
+[x `a`{.a](u)
+.
+<p><a href="u">x <code>a</code>{.a</a></p>
+.
+
+comment: inline code
+.
+`a`{%c}
+.
+<p><code>a</code></p>
+.
+
+comment: after a class
+.
+`a`{.a %c}
+.
+<p><code class="a">a</code></p>
+.
+
+comment: link
+.
+[a](u){%c}
+.
+<p><a href="u">a</a></p>
+.
+
+comment: closed by a brace, following text is kept
+.
+`a`{%a}b
+.
+<p><code>a</code>b</p>
+.
+
+comment: a brace inside a comment closed by a percent
+.
+`a`{% x } y %}
+.
+<p><code>a</code></p>
+.
+
+comment: attributes after a brace inside the comment
+.
+`a`{.a % c } % #i}
+.
+<p><code id="i" class="a">a</code></p>
+.
+
+comment: an empty comment containing a brace
+.
+`a`{% } %}
+.
+<p><code>a</code></p>
+.
+
+comment: text after a brace-closed comment is kept
+.
+`a`{.a %c} and {more}
+.
+<p><code class="a">a</code> and {more}</p>
+.
+
+comment: a later percent keeps a brace-closed comment open
+.
+`a`{.a %c} and 100% sure
+.
+<p><code>a</code>{.a %c} and 100% sure</p>
+.
+
 spans: simple
 .
 [a]{#id .b}c
@@ -176,6 +424,34 @@ spans: escaped span attribute
 <p>[a]{.b}</p>
 .
 
+spans: unterminated attributes are not a span
+.
+[a]{
+.
+<p>[a]{</p>
+.
+
+spans: unterminated attributes with a class are not a span
+.
+[a]{.x
+.
+<p>[a]{.x</p>
+.
+
+spans: comment
+.
+[a]{%c}
+.
+<p><span>a</span></p>
+.
+
+spans: a brace inside an unterminated comment is not a span
+.
+[a]{% a } %{b}
+.
+<p>[a]{% a } %{b}</p>
+.
+
 spans: nested text syntax
 .
 [*a*]{.b}c
@@ -238,7 +514,49 @@ spans: merge attributes
 .
 [a]{#a .a}{#b .a .b other=c}{other=d}
 .
-<p><span id="b" class="a b" other="d">a</span></p>
+<p><span id="b" class="a a b" other="d">a</span></p>
+.
+
+spans: merge classes from two groups
+.
+[a]{.x}{.y}
+.
+<p><span class="x y">a</span></p>
+.
+
+spans: merge classes from three groups
+.
+[a]{.x}{.y}{.z}
+.
+<p><span class="x y z">a</span></p>
+.
+
+spans: merge classes with an id in each group
+.
+[a]{.x #p}{.y #q}
+.
+<p><span id="q" class="x y">a</span></p>
+.
+
+spans: merge classes on the outer of nested spans
+.
+[[a]{.i}]{.x}{.y}
+.
+<p><span class="x y"><span class="i">a</span></span></p>
+.
+
+links: merge classes from two groups
+.
+[a](u){.x}{.y}
+.
+<p><a href="u" class="x y">a</a></p>
+.
+
+links: merge classes from three groups
+.
+[a](u){.x}{.y}{.z}
+.
+<p><a href="u" class="x y z">a</a></p>
 .
 
 Indented by 4 spaces