diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7c05460..cb9ad2a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ### Unreleased
 
+* Accept CSS `<number>` values with an omitted integer part (e.g. `.1`) inside `rgb()`/`rgba()`/`hsl()`/`hsla()`. Previously `RE_COLOUR_NUMERIC` and `RE_COLOUR_NUMERIC_ALPHA` required at least one digit before the decimal point, which caused colours such as `rgba(0,0,0,.1)` to be silently dropped during shorthand expansion (`background-color` from `background:`, `border-*-color` from `border:`).
+
 ### Version 2.1.0
 * Validate ssl when pulling files via https
 
diff --git a/lib/css_parser/regexps.rb b/lib/css_parser/regexps.rb
index cf83b2d..911570b 100644
--- a/lib/css_parser/regexps.rb
+++ b/lib/css_parser/regexps.rb
@@ -259,8 +259,10 @@ def self.regex_possible_values(*values)
     inherit
     currentColor
   ].freeze
-  RE_COLOUR_NUMERIC = /\b(hsl|rgb)\s*\(-?\s*-?\d+(\.\d+)?%?\s*%?,-?\s*-?\d+(\.\d+)?%?\s*%?,-?\s*-?\d+(\.\d+)?%?\s*%?\)/i.freeze
-  RE_COLOUR_NUMERIC_ALPHA = /\b(hsla|rgba)\s*\(-?\s*-?\d+(\.\d+)?%?\s*%?,-?\s*-?\d+(\.\d+)?%?\s*%?,-?\s*-?\d+(\.\d+)?%?\s*%?,-?\s*-?\d+(\.\d+)?%?\s*%?\)/i.freeze
+  # CSS <number> allows the integer part to be omitted (e.g. `.1`), per CSS Values & Units.
+  # `(?:\d*\.)?\d+` accepts `1`, `1.5`, and `.5` while still rejecting bare `1.`.
+  RE_COLOUR_NUMERIC = /\b(hsl|rgb)\s*\(-?\s*-?(?:\d*\.)?\d+%?\s*%?,-?\s*-?(?:\d*\.)?\d+%?\s*%?,-?\s*-?(?:\d*\.)?\d+%?\s*%?\)/i.freeze
+  RE_COLOUR_NUMERIC_ALPHA = /\b(hsla|rgba)\s*\(-?\s*-?(?:\d*\.)?\d+%?\s*%?,-?\s*-?(?:\d*\.)?\d+%?\s*%?,-?\s*-?(?:\d*\.)?\d+%?\s*%?,-?\s*-?(?:\d*\.)?\d+%?\s*%?\)/i.freeze
   RE_COLOUR_HEX = /\s*#([0-9a-fA-F]{6}|[0-9a-fA-F]{3})\b/.freeze
   RE_COLOUR_NAMED = /\s*\b(#{NAMED_COLOURS.join('|')})\b/i.freeze
   RE_COLOUR = Regexp.union(RE_COLOUR_NUMERIC, RE_COLOUR_NUMERIC_ALPHA, RE_COLOUR_HEX, RE_COLOUR_NAMED)
diff --git a/test/test_css_parser_regexps.rb b/test/test_css_parser_regexps.rb
index 6404017..1394d36 100644
--- a/test/test_css_parser_regexps.rb
+++ b/test/test_css_parser_regexps.rb
@@ -40,6 +40,7 @@ def test_colour
       'color: #fff', 'color:#f0a09c;', 'color: #04A', 'color: #04a9CE',
       'color: rgb(100, -10%, 300);', 'color: rgb(10,10,10)', 'color:rgb(12.7253%, -12%,0)',
       'color: hsla(-15, -77%, 19%, 5%);',
+      'color: rgba(0,0,0,.1)', 'color: rgba(0, 0, 0, .5)', 'color: hsla(0, 0%, 0%, .05)',
       'color: black', 'color:Red;', 'color: AqUa;', 'color: blue   ', 'color: transparent',
       'color: darkslategray'
     ].each do |colour|
diff --git a/test/test_rule_set_expanding_shorthand.rb b/test/test_rule_set_expanding_shorthand.rb
index ea7b719..e0b7476 100644
--- a/test/test_rule_set_expanding_shorthand.rb
+++ b/test/test_rule_set_expanding_shorthand.rb
@@ -18,6 +18,17 @@ def test_expanding_border_shorthand
     assert_equal '1px', declarations['border-top-width']
     assert_equal 'solid', declarations['border-bottom-style']
 
+    # Regression: rgba/hsla with no leading zero on the alpha (e.g. `.1`) used
+    # to fail the colour regex, causing border-*-color to be silently dropped
+    # during shorthand expansion.
+    declarations = expand_declarations('border: 1px solid rgba(0,0,0,.1)')
+    assert_equal '1px', declarations['border-top-width']
+    assert_equal 'solid', declarations['border-top-style']
+    assert_equal 'rgba(0,0,0,.1)', declarations['border-top-color']
+    assert_equal 'rgba(0,0,0,.1)', declarations['border-right-color']
+    assert_equal 'rgba(0,0,0,.1)', declarations['border-bottom-color']
+    assert_equal 'rgba(0,0,0,.1)', declarations['border-left-color']
+
     declarations = expand_declarations('border-color: red hsla(255, 0, 0, 5) rgb(2% ,2%,2%)')
     assert_equal 'red', declarations['border-top-color']
     assert_equal 'rgb(2%,2%,2%)', declarations['border-bottom-color']
@@ -197,7 +208,12 @@ def test_getting_background_size_from_shorthand
   end
 
   def test_getting_background_colour_from_shorthand
-    ['blue', 'lime', 'rgb(10,10,10)', 'rgb (  -10%, 99, 300)', '#ffa0a0', '#03c', 'trAnsparEnt', 'inherit'].each do |colour|
+    [
+      'blue', 'lime', 'rgb(10,10,10)', 'rgb (  -10%, 99, 300)', '#ffa0a0', '#03c', 'trAnsparEnt', 'inherit',
+      # Regression: alpha without a leading zero (e.g. `.1`) used to fail the
+      # colour regex and silently drop background-color.
+      'rgba(0,0,0,.1)'
+    ].each do |colour|
       shorthand = "background:#{colour} url('chess.png') center repeat fixed ;"
       declarations = expand_declarations(shorthand)
       assert_equal(colour, declarations['background-color'])