css change opacity of hex color code example

Example 1: hex color transparent

background-color: #ffffff80; <--- 80 is 50% opacity

/*r  g  b  a*/
# ff ff ff 80;

Converted values: 
100% — FF
99% — FC
98% — FA
97% — F7
96% — F5
95% — F2
94% — F0
93% — ED
92% — EB
91% — E8
90% — E6
89% — E3
88% — E0
87% — DE
86% — DB
85% — D9
84% — D6
83% — D4
82% — D1
81% — CF
80% — CC
79% — C9
78% — C7
77% — C4
76% — C2
75% — BF
74% — BD
73% — BA
72% — B8
71% — B5
70% — B3
69% — B0
68% — AD
67% — AB
66% — A8
65% — A6
64% — A3
63% — A1
62% — 9E
61% — 9C
60%99
59%96
58%94
57%91
56%8F
55% — 8C
54% — 8A
53%87
52%85
51%82
50%80
49%7D
48% — 7A
47%78
46%75
45%73
44%70
43% — 6E
42% — 6B
41%69
40%66
39%63
38%61
37% — 5E
36% — 5C
35%59
34%57
33%54
32%52
31%4F
30%4D
29% — 4A
28%47
27%45
26%42
25%40
24%3D
23% — 3B
22%38
21%36
20%33
19%30
18% — 2E
17% — 2B
16%29
15%26
14%24
13%21
12%1F
11% — 1C
10% — 1A
9%17
8%14
7%12
6%0F
5%0D
4% — 0A
3%08
2%05
1%03
0%00

Example 2: background opacity css hex

background-color: #ff000088; <--- the 88 is the alpha
background-color: #ff0000 50%;