It would be better to have the basic structure in the `layout.html.twig` and the contents of each block in separate blocks

like f.e:

```html
<!DOCTYPE html>
 <!--[if IEMobile 7 ]><html dir="ltr" lang="en-US"class="no-js iem7"> <![endif]--><!--[if lt IE 7]><html dir="ltr" lang="en-US" class="no-js ie6 oldie"> <![endif]--><!--[if IE 7]><html dir="ltr" lang="en-US" class="no-js ie7 oldie"><![endif]--><!--[if IE 8]><html dir="ltr" lang="en-US" class="no-js ie8 oldie"><![endif]--><!--[if (gte IE 9)|(gt IEMobile 7)|!(IEMobile)|!(IE)]><!--><html dir="ltr" lang="en-US" class="no-js"><!--<![endif]-->
 <head>
  {% block head %}
  ...
  {% endblock %}
 </head>
<body>
  <header>
    {% block header %}
    {% endblock %}
  </header>
  {% block content %}
  ...
  {% endblock %}
</body>
```

This way you only need to overwrite `header` in specific bundles and you can define other blocks as defaults in the layout.