A11y improvements and style tweaks

- Use button element instead of anchors, fixes keyboard navigation and
other issues
- Disable search input until it's open, prevents being able to write
inside it while it's closed
- Use :root instead of html in CSS
- Add minimal size for separator between home button and other links

templates/404.html

@@ -3,7 +3,7 @@
 {% block content %}
 <picture>
 	<source srcset="{{ get_url(path='404.png') }}" media="(prefers-reduced-motion: reduce)"></source>
-	<img id="not-found" class="pixels transparent no-hover" srcset="{{ get_url(path='404.gif') }}">
+	<img id="not-found" class="pixels transparent no-hover" alt="404" src="{{ get_url(path='404.gif') }}">
 </picture>
 
 <h1>Page Not Found</h1>
@@ -11,7 +11,7 @@
 <p>The requested page could not be found.{% if config.extra.issues_url %} If you feel this is not normal, then you can create an issue on the issue tracker.{% endif %}</p>
 
 <div class="dialog-buttons">
-	<a onclick="window.history.go(-1)" class="inline-button">Go Back</a>
+	<button class="inline-button" onclick="window.history.go(-1)">Go Back</button>
 	{%- if config.extra.issues_url %}
 		<a class="inline-button colored external" href="{{ config.extra.issues_url }}">File an Issue</a>
 	{%- endif %}