:root : represent root element of a tree in the document. In HTML root represents the <html> element.
Error message
Warning: Undefined array key "#submit_button" in Drupal\Core\Render\Element\Button::preRenderButton() (line 91 of core/lib/Drupal/Core/Render/Element/Button.php).
Drupal\Core\Render\Element\Button::preRenderButton() call_user_func_array() (Line: 107) Drupal\Core\Render\Renderer->doTrustedCallback() (Line: 876) Drupal\Core\Render\Renderer->doCallback() (Line: 438) Drupal\Core\Render\Renderer->doRender() (Line: 510) Drupal\Core\Render\Renderer->doRender() (Line: 510) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 92) __TwigTemplate_7a73f56f5318f4bbce87024a0c4b0513->block_content() (Line: 446) Twig\Template->yieldBlock() (Line: 71) __TwigTemplate_7a73f56f5318f4bbce87024a0c4b0513->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 57) __TwigTemplate_4bd1c0eb0cbe812b32e04c0e8f4586a9->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 100) __TwigTemplate_a507212c1d1f87295d8eeb597873d07e->block_content() (Line: 446) Twig\Template->yieldBlock() (Line: 79) __TwigTemplate_a507212c1d1f87295d8eeb597873d07e->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 510) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 51) __TwigTemplate_26ff4d3607963c25ef5d8d35d06625c5->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 98) __TwigTemplate_a07762ad7fd3af2e684dffdc448359ec->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 162) Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 627) Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 157) Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse() (Line: 90) Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray() (Line: 246) Symfony\Component\EventDispatcher\EventDispatcher::Symfony\Component\EventDispatcher\{closure}() (Line: 206) Symfony\Component\EventDispatcher\EventDispatcher->callListeners() (Line: 56) Symfony\Component\EventDispatcher\EventDispatcher->dispatch() (Line: 188) Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76) Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53) Drupal\Core\StackMiddleware\Session->handle() (Line: 48) Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28) Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32) Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 201) Drupal\page_cache\StackMiddleware\PageCache->fetch() (Line: 138) Drupal\page_cache\StackMiddleware\PageCache->lookup() (Line: 87) Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48) Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51) Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 53) Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51) Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 715) Drupal\Core\DrupalKernel->handle() (Line: 19)Warning: Undefined array key "#submit_button" in Drupal\Core\Render\Element\Button::preRenderButton() (line 91 of core/lib/Drupal/Core/Render/Element/Button.php).
Drupal\Core\Render\Element\Button::preRenderButton() call_user_func_array() (Line: 107) Drupal\Core\Render\Renderer->doTrustedCallback() (Line: 876) Drupal\Core\Render\Renderer->doCallback() (Line: 438) Drupal\Core\Render\Renderer->doRender() (Line: 510) Drupal\Core\Render\Renderer->doRender() (Line: 510) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 100) __TwigTemplate_a507212c1d1f87295d8eeb597873d07e->block_content() (Line: 446) Twig\Template->yieldBlock() (Line: 79) __TwigTemplate_a507212c1d1f87295d8eeb597873d07e->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 90) __TwigTemplate_2b094f9e97a47aee0f30ce002cee4213->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 100) __TwigTemplate_a507212c1d1f87295d8eeb597873d07e->block_content() (Line: 446) Twig\Template->yieldBlock() (Line: 79) __TwigTemplate_a507212c1d1f87295d8eeb597873d07e->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 510) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 155) __TwigTemplate_26ff4d3607963c25ef5d8d35d06625c5->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 491) Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 98) __TwigTemplate_a07762ad7fd3af2e684dffdc448359ec->doDisplay() (Line: 402) Twig\Template->yield() (Line: 358) Twig\Template->display() (Line: 373) Twig\Template->render() (Line: 51) Twig\TemplateWrapper->render() (Line: 34) twig_render_template() (Line: 380) Drupal\Core\Theme\ThemeManager->render() (Line: 497) Drupal\Core\Render\Renderer->doRender() (Line: 227) Drupal\Core\Render\Renderer->render() (Line: 162) Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 627) Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 157) Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse() (Line: 90) Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray() (Line: 246) Symfony\Component\EventDispatcher\EventDispatcher::Symfony\Component\EventDispatcher\{closure}() (Line: 206) Symfony\Component\EventDispatcher\EventDispatcher->callListeners() (Line: 56) Symfony\Component\EventDispatcher\EventDispatcher->dispatch() (Line: 188) Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76) Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53) Drupal\Core\StackMiddleware\Session->handle() (Line: 48) Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28) Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32) Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 201) Drupal\page_cache\StackMiddleware\PageCache->fetch() (Line: 138) Drupal\page_cache\StackMiddleware\PageCache->lookup() (Line: 87) Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48) Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51) Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 53) Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51) Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 715) Drupal\Core\DrupalKernel->handle() (Line: 19)
What is :root in CSS?