Chromeでは次のスタイルを使用できるはずです。
/* The entire area of the popup including area outside the bubble shape */
::-webkit-validation-bubble{}
/* Portion above the bubble behind top arrow */
::-webkit-validation-bubble-arrow-clipper{}
/* The arrow at the top of the bubble */
::-webkit-validation-bubble-arrow{}
/* The area containing the validation message */
::-webkit-validation-bubble-message{}
これは、このStackOverflowの質問で参照されています。
デフォルトのスタイルは次のとおりです。
/* form validation message bubble */
::-webkit-validation-bubble {
display: block;
z-index: 2147483647;
position: absolute;
opacity: 0.9;
line-height: 0;
-webkit-text-security: none;
-webkit-transition: opacity 05.5s ease;
}
::-webkit-validation-bubble-message {
display: block;
font: message-box;
min-width: 50px;
max-width: 200px;
border: solid 2px black;
background: -webkit-gradient(linear, left top, left bottom, from(#fbf9f9), to(#f0e4e4));
padding: 8px;
-webkit-border-radius: 8px;
-webkit-box-shadow: 4px 4px 4px rgba(204,204,204,0.7);
line-height: normal;
}
::-webkit-validation-bubble-top-outer-arrow {
display: inline-block;
position: relative;
left: 14px;
height: 0;
width: 0;
border-style: solid;
border-width: 14px;
border-bottom-color: black;
border-right-color: transparent;
border-top-width: 0;
border-left-width: 0;
}
::-webkit-validation-bubble-top-inner-arrow {
display: inline-block;
height: 0;
width: 0;
border-style: solid;
border-width: 10px; /* <border box width of outer-arrow> - <message border width> * 2 */
border-bottom-color: #fbf9f9;
border-right-color: transparent;
border-top-width: 0;
border-left-width: 0;
position: relative;
top: 2px; /* <message border width> */
left: 2px; /* <outer-arrow position> + <message border width> - <border box width of outer-arrow> */
}