CSS bug or wrong code in 'counter-increment'

Designing a new web page, I found strange behavior of the CSS 'counter-increment' styles.
I have a CSS file with code:

@charset "utf-8";
body ol {
	list-style-type: decimal;
	list-style-position: inside;
	counter-reset:level1;
}
ol ol {
	list-style-type: decimal;
	list-style-position: inside;
	counter-reset:level2;
}
ol ol ol {
	list-style-type: decimal;
	list-style-position: inside;
	counter-reset:level3;
}
ol ol ol ol {
	list-style-type: decimal;
	list-style-position: inside;
	counter-reset:level4;
}
ol ol ol ol ol {
	list-style-type: decimal;
	list-style-position: inside;
	counter-reset:level5;
}
li {
	display:block;
}
ol li:before {
	content: counters(level1, ".") ". ";
	counter-increment:level1;
}
ol ol li:before {
	content: counters(level1, ".") "." counters(level2, ".") ". ";
	counter-increment:level2;
}
ol ol ol li:before {
	content: counters(level1, ".") "." counters(level2, ".") "." counters(level3, ".") ". ";
	counter-increment:level3;
}
ol ol ol ol li:before {
	content: counters(level1, ".") "." counters(level2, ".") "." counters(level3, ".") "." counters(level4, ".") ". ";
	counter-increment:level4;
}
ol ol ol ol ol li:before {
	content: counters(level1, ".") "." counters(level2, ".") "." counters(level3, ".") "." counters(level4, ".") "." counters(level5, ".") ". ";
	counter-increment:level5;
}

and in HTML page I have a code like that:

<ol>
	<li>block 1</li>
	<li>block 2</li>
	<ol><li>subblock 1</li>
	    <li>subblock 2</li>
	    <ol><li>subblock 1</li>
		<li>subblock 2</li>
		<li>subblock 3</li>
		<li>subblock 4</li>
	    </ol>
	    <li>block 3</li>
	    <ol><li>subblock 1</li>
		<li>subblock 2</li>
		<li>subblock 3</li>
	    </ol>
	</ol>
</ol>

when I view this page in Opera, I have a strange result:
1. block 1
2. block 2
2.1. subblock 1
2.2. subblock 2
2.2.1. subblock 1
2.2.2. subblock 2
2.2.3. subblock 3
2.2.4. subblock 3
2.3. block 3
2.3.4.1. subblock 1
2.3.4.2. subblock 2
2.3.4.3. subblock 3

meanwhile in IE I see correct counting:
1. block 1
2. block 2
2.1. subblock 1
2.2. subblock 2
2.2.1. subblock 1
2.2.2. subblock 2
2.2.3. subblock 3
2.2.4. subblock 4
2.3. block 3
2.3.1. subblock 1
2.3.2. subblock 2
2.3.3. subblock 3

Is this Opera bug, or my code is wrong?

Your HTML markup is invalid and your CSS is unnecessarily complicated.

An OL element can only have LI elements as immediate children. You cannot have an OL as an immediate child of another OL like in your example; it has to be encapsulated within an LI.

Also, instead of using separate counters for each level, just use one and take advantage of how CSS handles counters on nested elements:

<ol>
  <li>block 1</li>
  <li>block 2
    <ol>
      <li>subblock 1</li>
      <li>subblock 2
        <ol>
          <li>subblock1</li>
          <li>subblock2</li>
          <li>subblock3</li>
          <li>subblock4</li>
        </ol>
      </li>
      <li>subblock 3
        <ol>
          <li>subblock1</li>
          <li>subblock2</li>
          <li>subblock3</li>
        </ol>
      </li>
  </li>
</ol>

ol {
  list-style-type:none;
  margin:0;
  padding:0;
  counter-reset:item;
}

ol li {
  counter-increment:item;
}

ol li:before {
  padding-right:0.5em;
  content:counters(item, ".") ".";
}

Thank you for help!
I will be harder learn CSS and HTML.