fix(curriculum): css and html test issues (#42045)

* fix: css and html issues

* fix: issues with css test

* fix: allow spaces in solution css

* fix: css tests for use-rgb-to-mix-colors

curriculum/challenges/english/04-data-visualization/data-visualization-with-d3/add-a-hover-effect-to-a-d3-element.md

@@ -21,7 +21,7 @@ Use the `attr()` method to add a class of `bar` to all the `rect` elements. This
 Your `rect` elements should have a class of `bar`.
 
 ```js
-assert($('rect').attr('class') == 'bar');
+assert($('rect').attr('class').trim().split(/\s+/g).includes('bar'));
 ```
 
 # --seed--

curriculum/challenges/english/04-data-visualization/data-visualization-with-d3/add-classes-with-d3.md

@@ -27,7 +27,7 @@ Add the `attr()` method to the code in the editor and put a class of `bar` on th
 Your `div` elements should have a class of `bar`.
 
 ```js
-assert($('div').attr('class') == 'bar');
+assert($('div').attr('class').trim().split(/\s+/g).includes('bar'));
 ```
 
 Your code should use the `attr()` method.