Set explicit opacity value on placeholder pseudo-element (#3308)

Fixes #3300.

Author: adamwathan

__tests__/fixtures/tailwind-output-flagged.css

@@ -439,6 +439,7 @@ textarea {
 
 input::placeholder,
 textarea::placeholder {
+  opacity: 1;
   color: #9ca3af;
 }
 

__tests__/fixtures/tailwind-output-important.css

@@ -439,6 +439,7 @@ textarea {
 
 input::placeholder,
 textarea::placeholder {
+  opacity: 1;
   color: #9ca3af;
 }
 

__tests__/fixtures/tailwind-output-no-color-opacity.css

@@ -439,6 +439,7 @@ textarea {
 
 input::placeholder,
 textarea::placeholder {
+  opacity: 1;
   color: #9ca3af;
 }
 

__tests__/fixtures/tailwind-output.css

@@ -439,6 +439,7 @@ textarea {
 
 input::placeholder,
 textarea::placeholder {
+  opacity: 1;
   color: #9ca3af;
 }
 

src/plugins/css/preflight.css

@@ -141,6 +141,7 @@ textarea {
 
 input::placeholder,
 textarea::placeholder {
+  opacity: 1;
   color: theme('colors.gray.400', #a1a1aa);
 }