protected function compileKeyframeBlock(Block $block, $selectors)
    {
        $env = $this->pushEnv($block);

        $envs = $this->compactEnv($env);

        $this->env = $this->extractEnv(array_filter($envs, function (Environment $e) {
            return ! isset($e->block->selectors);
        }));

        $this->scope = $this->makeOutputBlock($block->type, $selectors);
        $this->scope->depth = 1;
        $this->scope->parent->children[] = $this->scope;

        $this->compileChildrenNoReturn($block->children, $this->scope);

        $this->scope = $this->scope->parent;
        $this->env   = $this->extractEnv($envs);

        $this->popEnv();
    }