Troubleshooting of strange issues such as css invalidation related to codePage = "936" in Asp.net

Recently I have a simple project on hand, which is much simpler than what I usually write, but it is not smooth. There are many strange problems, such as a strange problem today, here I will talk about the whole process of solution.
The customer requested that a TextBox should be unavailable when the second item is selected in DropDownList. This is a simple problem. Set the AutoPostBack attribute of DropDownList to "True ", then write the code in the SelectedIndexChanged event of DropDownList:
If (ddlFrame. SelectedIndex = 0) // if the first item is selected
     {
TxtFrameID. Enabled = true;
     }
Else
     {
TxtFrameID. Text = "";
TxtFrameID. Enabled = false;
     }
If it is normal, this will solve the problem, but today there is a strange problem, no matter how I choose DropDownList, it selects the first item after refreshing, how can I choose not to select the second item, at first, I thought it was DropDownList's EnableViewState attribute which was set to false. The result showed that the attribute value was true. This is strange. How can I choose not to select the second item?
So I opened the previously written normal page in notepad to compare the differences between the two pages. I found a suspicious line at the top of the problematic page:
<% @ Page language = "c #" Codebehind = "BookInfo. Aspx. cs" AutoEventWireup = "false" Inherits = "WebS. BookInfo" codePage = "936" %>
You must have discovered that, in the end, there is a sentence: codePage = "936", what is this? I checked the information and found the conclusion:
<% @ Codepage = 936%> Simplified Chinese
<% @ Codepage = 950%> Traditional Chinese
<% @ Codepage = 65001%> UTF-8
CodePage = "936" indicates simplified Chinese, so I deleted it and found that DropDownList can select the second item, but the new problem came out again: css seems to have expired, the page becomes messy. so I switched to VS2003. In the page properties, set the character set of the page to GB2312. After saving the settings, I found that the page is normal again. Then, how can I choose the drop-down menu to stop on the first one, I suddenly remembered, web. the config file also has a language set, so I will