Bloc video : correction a11y (#769)

layouts/partials/blocks/templates/video.html

 {{- $block := .block -}}
 {{- $block_class := partial "GetBlockClass" .block -}}
 {{- $block_index := .index -}}
+{{- $described_by_id := "" -}}
 
 {{- with .block.data -}}
   <div class="{{ $block_class }}">
@@ -28,11 +29,13 @@
         {{ end }}
 
         {{ if .video_title }}
-          <p>{{- partial "PrepareHTML" .video_title -}}</p>
+          {{ $described_by_id = printf "block-video-title-%d" $block_index }}
+          <p aria-hidden="true">{{- partial "PrepareHTML" .video_title -}}</p>
         {{ end }}
 
         {{ partial "commons/transcription" ( dict
           "block_index" $block_index
+          "described_by" $described_by_id
           "transcription" .transcription
         ) }}
       </div>

layouts/partials/blocks/templates/video/lazy-container.html

 <div class="lazy-video-player">
-  <button class="lazy-video-player__play" title="{{- i18n "blocks.video.play" -}}" aria-label="{{- i18n "blocks.video.play" -}}"></button>
+  <button class="lazy-video-player__play">
+    <span class="sr-only">{{- i18n "blocks.video.play" }} - {{ .title -}}</span>
+  </button>
   {{ if .poster }}
-    <img src="{{ .poster }}" alt="{{ .title }}" loading="lazy">
+    <img src="{{ .poster }}" alt="" loading="lazy">
   {{ end }}
 </div>
 <div class="video-container">