原创！转载清注明出自http://blog.csdn.net/lichwei1983
1 支持焦点
W3C规定，只有web page明确设置了tabIndex的Element才定义为支持焦点，他是这么定义一个Node是否支持焦点的：
bool Node::supportsFocus() const
{
return hasRareData() && rareData()->tabIndexSetExplicitly();
}
2 可被设置焦点
首先，Node必须支持焦点，其次，此Node的render对象RenderBox必须存在，而且此RenderBox的style属性设置为可见
bool Node::isFocusable() const
{
if (!inDocument() || !supportsFocus())
return false;
if (renderer())
ASSERT(!renderer()->needsLayout());
else
// If the node is in a display:none tree it might say it needs style recalc but
// the whole document is actually up to date.
ASSERT(!document()->childNeedsStyleRecalc());
// FIXME: Even if we are not visible, we might have a child that is visible.
// Hyatt wants to fix that some day with a "has visible content" flag or the like.
if (!renderer() || renderer()->style()->visibility() != VISIBLE)
return false;
return true;
}
3 按键可获取焦点
一个Element能否获取焦点，取决于：
1）自己“可被设置焦点”
2）tabIndex是正整数
bool Node::isKeyboardFocusable(KeyboardEvent*) const
{
return isFocusable() && tabIndex() >= 0;
}
4 鼠标可获取焦点
只需要判断“可被获取焦点”就行了：
bool Node::isMouseFocusable() const
{
return isFocusable();
}