Refactor avoidsFloats() to reflect CSS2.1 spec more clearly

LayoutTests/fast/block/float/block-formatting-context-crash.html

+<script> 
+if (window.testRunner) {
+    testRunner.waitUntilDone();
+    testRunner.dumpAsText();
+}
+</script>
+<style>
+.fixed:nth-last-child(even) { display: table-column; position: fixed; }
+.float { float: left; border-style: ridge; }
+</style>
+<script>
+function crash(caption, thead) {
+    caption.appendChild(thead);
+    if (window.testRunner)
+        testRunner.notifyDone();
+}
+
+function boom() {
+    var section = document.createElement('section');
+    section.setAttribute('class', 'fixed');
+    document.documentElement.appendChild(section);
+    var input = document.createElement('input');
+    document.documentElement.appendChild(input);
+    var input2 = document.createElement('input');
+    document.documentElement.appendChild(input2);
+    var table = document.createElement('table');
+    document.documentElement.appendChild(table);
+    var tbody = document.createElement('tbody');
+    document.documentElement.appendChild(tbody);
+    var thead = document.createElement('thead');
+    thead.setAttribute('class', 'float');
+    var ul = document.createElement('ul');
+    document.documentElement.appendChild(ul);
+    var caption = document.createElement('caption');
+    document.documentElement.appendChild(caption);
+    setTimeout(function() { ul.appendChild(table); }, 50);
+    section.appendChild(thead);
+    setTimeout(crash(caption, thead), 60);
+}
+window.onload = boom;
+</script>
+<p> The test should not crash.</p>