minor fixes

1-js/02-first-steps/05-types/article.md

@@ -46,13 +46,15 @@ Besides regular numbers, there are so-called "special numeric values" which also
     alert( "not a number" / 2 ); // NaN, such division is erroneous
     ```
 
-    `NaN` is sticky. Any further operation on `NaN` returns `NaN`:
+    `NaN` is sticky. Any further mathematical operation on `NaN` returns `NaN`:
 
     ```js run
-    alert( "not a number" / 2 + 5 ); // NaN
+    alert( NaN + 1 ); // NaN
+    alert( 3 * NaN ); // NaN
+    alert( "not a number" / 2 - 1 ); // NaN
     ```
 
-    So, if there's a `NaN` somewhere in a mathematical expression, it propagates to the whole result.
+    So, if there's a `NaN` somewhere in a mathematical expression, it propagates to the whole result (there's only one exception to that: `NaN ** 0` is `1`).
 
 ```smart header="Mathematical operations are safe"
 Doing maths is "safe" in JavaScript. We can do anything: divide by zero, treat non-numeric strings as numbers, etc.

9-regular-expressions/14-regexp-lookahead-lookbehind/article.md

@@ -59,6 +59,10 @@ alert( str.match(/\d+\b(?!€)/g) ); // 2 (the price is not matched)
 
 ## Lookbehind
 
+```warn header="Lookbehind browser compatibility"
+Please Note: Lookbehind is not supported in non-V8 browsers, such as Safari, Internet Explorer.
+```
+
 Lookahead allows to add a condition for "what follows".
 
 Lookbehind is similar, but it looks behind. That is, it allows to match a pattern only if there's something before it.