Fix build warnings by using DOM std capitalization

microsite/blog/2020-09-08-announcing-tech-docs.md

@@ -6,7 +6,7 @@ authorURL: https://github.com/garyniemen
 
 Since we [open sourced Backstage](https://backstage.io/blog/2020/03/16/announcing-backstage), one of the most requested features has been for a technical documentation plugin. Well, good news. The first open source version of TechDocs is here. Now let’s start collaborating and making it better, together.
 
-<iframe width="780" height="440" src="https://www.youtube.com/embed/mOLCgdPw1iA" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen>
+<iframe width="780" height="440" src="https://www.youtube.com/embed/mOLCgdPw1iA" frameBorder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowFullScreen>
 </iframe>
 
 <!--truncate-->

microsite/blog/2020-10-22-cost-insights-plugin.md

@@ -6,7 +6,7 @@ authorURL: https://twitter.com/janisa_a
 
 How did Spotify save millions on cloud costs within a matter of months?? We made cost optimization just another part of the daily development process. Our newly open sourced [Cost Insights plugin](https://github.com/backstage/backstage/tree/master/plugins/cost-insights) makes a team’s cloud costs visible — and actionable — right inside Backstage. So engineers can see the impact of their cloud usage (down to a product and resource level) and make optimizations wherever and whenever it makes sense. By managing cloud costs from the ground up, you can make smarter decisions that let you continue to build and scale quickly, without wasting resources.
 
-<iframe width="780" height="440" src="https://www.youtube.com/embed/YLAd5hdXR_Q" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+<iframe width="780" height="440" src="https://www.youtube.com/embed/YLAd5hdXR_Q" frameBorder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowFullScreen></iframe>
 
 Are we turning engineers into accountants? Nope, we’re just letting engineers do what they do best, in the place that feels natural to them: inside Backstage.
 

microsite/pages/en/demos.js

@@ -100,7 +100,7 @@ const Background = props => {
               width="560"
               height="315"
               src="https://www.youtube.com/embed/YLAd5hdXR_Q"
-              frameborder="0"
+              frameBorder="0"
               allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture"
               allowfullscreen
             ></iframe>
@@ -135,7 +135,7 @@ const Background = props => {
               width="560"
               height="315"
               src="https://www.youtube.com/embed/mOLCgdPw1iA"
-              frameborder="0"
+              frameBorder="0"
               allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture"
               allowfullscreen
             ></iframe>